android: add ThumbnailCache

chrome/browser/android/thumbnail_cache.cc

Index: chrome/browser/android/thumbnail_cache.cc
diff --git a/chrome/browser/android/thumbnail_cache.cc b/chrome/browser/android/thumbnail_cache.cc
new file mode 100644
index 0000000000000000000000000000000000000000..2bb8cf742e90a0cf68ba7e568058de7c1d93c033
--- /dev/null
+++ b/chrome/browser/android/thumbnail_cache.cc
@@ -0,0 +1,1357 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/android/thumbnail_cache.h"
+
+#include <algorithm>
+#include <cmath>
+
+#include "base/file_util.h"
+#include "base/files/file.h"
+#include "base/files/file_enumerator.h"
+#include "base/files/file_path.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/time/time.h"
+#include "chrome/browser/android/tab_thumbnail_provider.h"
+#include "content/public/browser/android/content_view_core.h"
+#include "content/public/browser/android/ui_resource_provider.h"
+#include "content/public/browser/browser_thread.h"
+#include "content/public/browser/render_view_host.h"
+#include "content/public/browser/render_widget_host_view.h"
+#include "content/public/browser/web_contents.h"
+#include "skia/ext/refptr.h"
+#include "third_party/android_opengl/etc1/etc1.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+#include "third_party/skia/include/core/SkCanvas.h"
+#include "third_party/skia/include/core/SkData.h"
+#include "third_party/skia/include/core/SkMallocPixelRef.h"
+#include "third_party/skia/include/core/SkPixelRef.h"
+#include "ui/gfx/geometry/size_conversions.h"
+
+namespace {
+
+const size_t kMaxReadbacks = 1;
+const bool kDropCachedNTPOnLowMemory = true;
+const float kApproximationScaleFactor = 4.f;
+const bool kEnableCompression = true;
+const base::TimeDelta kCaptureMinRequestTimeMs(
+    base::TimeDelta::FromMilliseconds(1000));
+const int kCompressedKey = 0xABABABAB;
+const int kDecompressedKey = 0xCDCDCDCD;
+
+// ETC1 texture sizes are multiples of four.
+size_t NextETC1Size(size_t s) {
+  return (s / 4 + (s % 4 ? 1 : 0)) * 4;
+}
+
+gfx::Size GetEncodedSize(gfx::Size bitmap_size) {
+  return gfx::Size(NextETC1Size(bitmap_size.width()),
+                   NextETC1Size(bitmap_size.height()));
+}
+
+}  // anonymous namespace
+
+ThumbnailCache::ThumbnailCache(const std::string& disk_cache_path_str,
+                               size_t default_cache_size,
+                               size_t approximation_cache_size,
+                               size_t compression_queue_max_size,
+                               size_t write_queue_max_size,
+                               bool use_approximation_thumbnails,
+                               float thumbnail_scale)
+    : ui_resource_provider_(NULL),
+      disk_cache_path_(disk_cache_path_str),
+      compression_queue_max_size_(compression_queue_max_size),
+      write_queue_max_size_(write_queue_max_size),
+      cache_(default_cache_size),
+      approximation_cache_(approximation_cache_size),
+      use_approximation_thumbnails_(use_approximation_thumbnails),
+      thumbnail_scale_(thumbnail_scale),
+      compression_thread_("thumbnail_compression"),
+      weak_factory_(this) {
+  compression_thread_.Start();
+}
+
+ThumbnailCache::~ThumbnailCache() {
+  compression_thread_.Stop();
+  SetUIResourceProvider(NULL);
+}
+
+void ThumbnailCache::SetUIResourceProvider(
+    content::UIResourceProvider* ui_resource_provider) {
+  if (ui_resource_provider_ == ui_resource_provider)
+    return;
+
+  // Clean up the UI resources.
+  for (ExpiringThumbnailCache::iterator iter = approximation_cache_.begin();
+       iter != approximation_cache_.end();
+       iter++) {
+    scoped_refptr<Thumbnail> thumbnail = iter->second;
+    thumbnail->CleanupThumbnail();
+  }
+
+  for (ExpiringThumbnailCache::iterator iter = cache_.begin();
+       iter != cache_.end();
+       iter++) {
+    scoped_refptr<Thumbnail> thumbnail = iter->second;
+    thumbnail->CleanupThumbnail();
+  }
+
+  if (ui_resource_provider_)
+    ui_resource_provider_->RemoveListener(this);
+
+  ui_resource_provider_ = ui_resource_provider;
+
+  if (ui_resource_provider_)
+    ui_resource_provider_->AddListener(this);
+}
+
+void ThumbnailCache::CacheInTab(TabId tab_id) {
+  GetThumbnail(tab_id, true);
+}
+
+void ThumbnailCache::AddThumbnailChangeListener(
+    ThumbnailChangeListener* listener) {
+  if (thumbnail_change_listeners_.find(listener) ==
+      thumbnail_change_listeners_.end()) {
+    thumbnail_change_listeners_.insert(listener);
+  }
+}
+
+void ThumbnailCache::RemoveThumbnailChangeListener(
+    ThumbnailChangeListener* listener) {
+  ThumbnailChangeListenerSet::iterator iter =
+      thumbnail_change_listeners_.find(listener);
+  if (iter != thumbnail_change_listeners_.end())
+    thumbnail_change_listeners_.erase(iter);
+}
+
+scoped_refptr<ThumbnailCache::Thumbnail> ThumbnailCache::GetThumbnail(
+    TabId id) {
+  return GetThumbnail(id, false);
+}
+
+scoped_refptr<ThumbnailCache::Thumbnail> ThumbnailCache::GetThumbnail(
+    TabId id,
+    bool cache_in_if_missing) {
+  scoped_refptr<Thumbnail> thumbnail = cache_.Get(id);
+
+  if (!thumbnail) {
+    if (cache_in_if_missing) {
+      AddIdToVisibleIds(id);
+      ReadNextThumbnail(id);
+    }
+    thumbnail = approximation_cache_.Get(id);
+  }
+
+  return thumbnail;
+}
+
+bool ThumbnailCache::CanReadTabContent(const TabThumbnailProvider* tab) {
+  content::ContentViewCore* view = tab->GetContentViewCore();
+  return view &&
+         view->GetWebContents()->GetRenderViewHost()->CanCopyFromBackingStore();
+}
+
+void ThumbnailCache::CacheTabThumbnail(const TabThumbnailProvider* tab) {
+  TabId tab_id = tab->GetAndroidId();
+  if (pending_thumbnail_readbacks_.find(tab_id) !=
+          pending_thumbnail_readbacks_.end() ||
+      pending_thumbnail_readbacks_.size() >= kMaxReadbacks) {
+    return;
+  }
+
+  InvalidateIfChanged(tab);
+  if (CheckAndUpdateThumbnailMetaData(tab, false)) {
+    base::Callback<void(int, const ThumbnailBitmap&)> end_callback = base::Bind(
+        &ThumbnailCache::EndCacheTabThumbnail, weak_factory_.GetWeakPtr());
+    scoped_refptr<TabReadbackRequest> readback =
+        make_scoped_refptr(new TabReadbackRequest(
+            tab_id, tab->GetContentViewCore(), thumbnail_scale_, end_callback));
+    pending_thumbnail_readbacks_[tab_id] = readback;
+    readback->Run();
+  }
+}
+
+void ThumbnailCache::CacheTabThumbnailWithBitmap(
+    const TabThumbnailProvider* tab,
+    const SkBitmap& bitmap,
+    float thumbnail_scale) {
+  bool is_native_page = true;
+  TabId tab_id = tab->GetAndroidId();
+  InvalidateIfChanged(tab);
+  if (CheckAndUpdateThumbnailMetaData(tab, is_native_page))
+    PutThumbnail(tab_id, ThumbnailBitmap(bitmap, thumbnail_scale));
+}
+
+void ThumbnailCache::EndCacheTabThumbnail(TabId tab_id,
+                                          const ThumbnailBitmap& bitmap) {
+  bool is_native_page = false;
+  TabReadbackRequestMap::iterator readback_iter =
+      pending_thumbnail_readbacks_.find(tab_id);
+  if (readback_iter != pending_thumbnail_readbacks_.end()) {
+    scoped_refptr<TabReadbackRequest> readback = readback_iter->second;
+    pending_thumbnail_readbacks_.erase(tab_id);
+    if (!readback->drop_after_readback() && bitmap.IsValid()) {
+      PutThumbnail(tab_id, bitmap);
+    }
+    NotifyListenersOfThumbnailChange(tab_id, is_native_page);
+  }
+}
+
+void ThumbnailCache::InvalidateIfChanged(const TabThumbnailProvider* tab) {
+  TabId tab_id = tab->GetAndroidId();
+  ThumbnailMetaDataMap::iterator meta_data_iter =
+      thumbnail_meta_data_.find(tab_id);
+  if (meta_data_iter == thumbnail_meta_data_.end()) {
+    // We need this case so we have metadata when loading the thumbnail from
+    // disk. Note that the time stamp is 0 to force invalidation when a new
+    // thumbnail is requested to be captured.
+    thumbnail_meta_data_[tab_id] =
+        ThumbnailMetaData(base::Time(), tab->GetURL());
+  } else if (meta_data_iter->second.url() != tab->GetURL()) {
+    Remove(tab_id);
+  }
+}
+
+void ThumbnailCache::PutThumbnail(TabId tab_id,
+                                  const ThumbnailBitmap& thumbnail_bitmap) {
+  if (!ui_resource_provider_ || !thumbnail_bitmap.IsValid())
+    return;
+
+  scoped_refptr<Thumbnail> thumbnail =
+      make_scoped_refptr(new Thumbnail(tab_id,
+                                       thumbnail_bitmap.bitmap(),
+                                       thumbnail_bitmap.scale(),
+                                       ui_resource_provider_));
+  PutThumbnail(tab_id, thumbnail_bitmap, thumbnail);
+}
+
+void ThumbnailCache::PutThumbnail(TabId tab_id,
+                                  const ThumbnailBitmap& thumbnail_bitmap,
+                                  scoped_refptr<Thumbnail> thumbnail) {
+  if (!ui_resource_provider_ || !thumbnail || !thumbnail_bitmap.IsValid())
+    return;
+
+  scoped_refptr<Thumbnail> approx_thumbnail = NULL;
+  ThumbnailBitmap approx_thumbnail_bitmap;
+  if (use_approximation_thumbnails_)
+    approx_thumbnail_bitmap = thumbnail_bitmap.CreateApproximation();
+
+  if (approx_thumbnail_bitmap.IsValid()) {
+    approx_thumbnail =
+        make_scoped_refptr(new ApproxThumbnail(tab_id,
+                                               approx_thumbnail_bitmap.bitmap(),
+                                               approx_thumbnail_bitmap.scale(),
+                                               ui_resource_provider_));
+  }
+
+  AddIdToVisibleIds(tab_id);
+  RemoveFromQueues(tab_id);
+  MakeSpaceForNewItemIfNecessary(tab_id);
+  CleanupThumbnail(cache_.Get(tab_id));
+  cache_.Put(tab_id, thumbnail);
+
+  if (approx_thumbnail) {
+    CleanupThumbnail(approximation_cache_.Get(tab_id));
+    approximation_cache_.Put(tab_id, approx_thumbnail);
+  }
+
+  CompressThumbnailIfNecessary(thumbnail);
+  WriteThumbnailIfNecessary(thumbnail);
+  NotifyListenersOfThumbnailChange(tab_id, true);
+}
+
+bool ThumbnailCache::CheckAndUpdateThumbnailMetaData(
+    const TabThumbnailProvider* tab,
+    bool is_native_page) {
+  if (!is_native_page && !CanReadTabContent(tab))
+    return false;
+
+  // TODO(tedchoc): Add check to see if the tab has actually drawn content.
+  // Then add a draw  counter to see if the content has actually changed since
+  // last thumbnail capture.
+  TabId tab_id = tab->GetAndroidId();
+  ThumbnailMetaDataMap::iterator meta_data_iter =
+      thumbnail_meta_data_.find(tab_id);
+  if (meta_data_iter != thumbnail_meta_data_.end() &&
+      meta_data_iter->second.url() == tab->GetURL() &&
+      (CurrentTime() - meta_data_iter->second.capture_time()) <
+          kCaptureMinRequestTimeMs) {
+    return false;
+  }
+
+  thumbnail_meta_data_[tab_id] =
+      ThumbnailMetaData(CurrentTime(), tab->GetURL());
+  return true;
+}
+
+void ThumbnailCache::RemoveFromCache(ExpiringThumbnailCache& cache,
+                                     const TabIdList& tab_ids) {
+  for (TabIdList::const_iterator iter = tab_ids.begin(); iter != tab_ids.end();
+       iter++) {
+    cache.Remove(*iter);
+  }
+}
+
+void ThumbnailCache::RemoveThumbnailsAndScheduleReload() {
+  TabIdList modified_entries;
+  TabIdList remove_from_cache;
+  for (ExpiringThumbnailCache::iterator iter = approximation_cache_.begin();
+       iter != approximation_cache_.end();
+       iter++) {
+    TabId tab_id = iter->first;
+    scoped_refptr<Thumbnail> thumbnail = iter->second;
+    thumbnail->CleanupThumbnail();
+
+    if (!thumbnail->AttemptToScheduleRebuildFromData())
+      remove_from_cache.push_back(tab_id);
+    modified_entries.push_back(tab_id);
+  }
+
+  read_queue_.clear();
+  RemoveFromCache(approximation_cache_, remove_from_cache);
+  remove_from_cache.clear();
+
+  for (ExpiringThumbnailCache::iterator iter = cache_.begin();
+       iter != cache_.end();
+       iter++) {
+    TabId tab_id = iter->first;
+    scoped_refptr<Thumbnail> thumbnail = iter->second;
+    thumbnail->CleanupThumbnail();
+
+    if (!thumbnail->AttemptToScheduleRebuildFromData())
+      remove_from_cache.push_back(tab_id);
+    modified_entries.push_back(tab_id);
+  }
+
+  RemoveFromCache(cache_, remove_from_cache);
+  remove_from_cache.clear();
+
+  for (TabIdList::iterator iter = modified_entries.begin();
+       iter != modified_entries.end();
+       iter++) {
+    NotifyListenersOfThumbnailChange(*iter, false);
+  }
+
+  ReadNextThumbnail();
+
+  TabIdList last_visible_ids = last_visible_ids_;
+  last_visible_ids_.clear();
+  UpdateVisibleIds(last_visible_ids);
+}
+
+void ThumbnailCache::UpdateVisibleIds(const TabIdList& priority) {
+  if (priority.empty()) {
+    last_visible_ids_.clear();
+    return;
+  }
+
+  size_t ids_size = std::min(priority.size(), cache_.MaximumCacheSize());
+  if (last_visible_ids_.size() == ids_size) {
+    // Early out if called with the same input as last time (We only care
+    // about the first mCache.MaximumCacheSize() entries).
+    bool lists_differ = false;
+    for (TabIdList::const_iterator visible_iter = last_visible_ids_.begin(),
+                                   priority_iter = priority.begin();
+         visible_iter != last_visible_ids_.end() &&
+             priority_iter != priority.end();
+         visible_iter++, priority_iter++) {
+      if (*priority_iter != *visible_iter) {
+        lists_differ = true;
+        break;
+      }
+    }
+    if (!lists_differ)
+      return;
+  }
+
+  last_visible_ids_.clear();
+  visible_ids_.clear();
+  size_t count = 0;
+  for (TabIdList::const_iterator iter = priority.begin();
+       iter != priority.end() && count < ids_size;
+       iter++, count++) {
+    TabId tab_id = *iter;
+    last_visible_ids_.push_back(tab_id);
+    visible_ids_.push_back(tab_id);
+  }
+
+  read_queue_.clear();
+
+  count = 0;
+  for (TabIdList::const_iterator iter = priority.begin();
+       iter != priority.end() && count < ids_size;
+       iter++, count++) {
+    TabId tab_id = *iter;
+    if (!cache_.Get(tab_id) &&
+        std::find(read_queue_.begin(), read_queue_.end(), tab_id) ==
+            read_queue_.end()) {
+      read_queue_.push_back(tab_id);
+    }
+  }
+
+  ReadNextThumbnail();
+}
+
+void ThumbnailCache::Remove(TabId tab_id) {
+  TabReadbackRequestMap::iterator readback_iter =
+      pending_thumbnail_readbacks_.find(tab_id);
+  if (readback_iter != pending_thumbnail_readbacks_.end())
+    readback_iter->second->SetDropAfterReadback(true);
+
+  scoped_refptr<Thumbnail> thumbnail = cache_.Remove(tab_id);
+  CleanupThumbnail(thumbnail);
+
+  scoped_refptr<Thumbnail> approx_thumbnail =
+      approximation_cache_.Remove(tab_id);
+  CleanupThumbnail(approx_thumbnail);
+
+  thumbnail_meta_data_.erase(tab_id);
+  RemoveFromDisk(tab_id);
+  RemoveFromQueues(tab_id);
+
+  if (thumbnail || approx_thumbnail)
+    NotifyListenersOfThumbnailChange(tab_id, false);
+}
+
+void ThumbnailCache::HandleLowMemory(bool consider_gpu_memory) {
+  HandleLowMemoryOnQueue(compression_queue_, consider_gpu_memory);
+  HandleLowMemoryOnQueue(write_queue_, consider_gpu_memory);
+
+  read_queue_.clear();
+
+  size_t cached_tabs_free_count = 0;
+  if (cache_.size() > 0) {
+    cached_tabs_free_count = std::max(cache_.size() / 2, 1u);
+    for (size_t i = 0; i < cached_tabs_free_count; ++i) {
+      RemoveThumbnailFromCache();
+    }
+  }
+}
+
+size_t ThumbnailCache::HandleLowMemoryOnQueue(ThumbnailQueue& queue,
+                                              bool consider_gpu_memory) {
+  size_t thumbnails_dropped = 0;
+  ThumbnailQueue next_queue;
+  for (ThumbnailQueue::iterator iter = queue.begin(); iter != queue.end();
+       iter++) {
+    scoped_refptr<Thumbnail> thumbnail = *iter;
+    TabId tab_id = thumbnail->tab_id();
+    if (!consider_gpu_memory && thumbnail.get() == cache_.Get(tab_id).get())
+      RebuildThumbnail(thumbnail, true);
+
+    CleanupThumbnail(thumbnail);
+    ++thumbnails_dropped;
+  }
+
+  queue = next_queue;
+
+  return thumbnails_dropped;
+}
+
+void ThumbnailCache::RemoveFromQueues(TabId tab_id) {
+  TabIdList::iterator read_iter =
+      std::find(read_queue_.begin(), read_queue_.end(), tab_id);
+  if (read_iter != read_queue_.end())
+    read_queue_.erase(read_iter);
+
+  ThumbnailQueue new_write_queue;
+  for (ThumbnailQueue::iterator iter = write_queue_.begin();
+       iter != write_queue_.end();
+       iter++) {
+    scoped_refptr<Thumbnail> write_thumbnail = *iter;
+    if (write_thumbnail->tab_id() == tab_id) {
+      write_thumbnail->CleanupCPUData(true);
+      CleanupThumbnail(write_thumbnail);
+    } else {
+      new_write_queue.push_back(write_thumbnail);
+    }
+  }
+  write_queue_ = new_write_queue;
+
+  ThumbnailQueue new_compression_queue;
+  for (ThumbnailQueue::iterator iter = compression_queue_.begin();
+       iter != compression_queue_.end();
+       iter++) {
+    scoped_refptr<Thumbnail> compress_thumbnail = *iter;
+    if (compress_thumbnail->tab_id() == tab_id) {
+      compress_thumbnail->CleanupCPUData(true);
+      CleanupThumbnail(compress_thumbnail);
+    } else {
+      new_compression_queue.push_back(compress_thumbnail);
+    }
+  }
+  compression_queue_ = new_compression_queue;
+}
+
+bool ThumbnailCache::WriteThumbnailIfNecessary(
+    scoped_refptr<Thumbnail> thumbnail) {
+  if (!thumbnail->DataWriteRequired())
+    return false;
+
+  if (RemoveDuplicateIdsFromQueueHelper(write_queue_, thumbnail))
+    return true;
+
+  if (write_queue_.size() > write_queue_max_size_) {
+    CleanupThumbnail(write_queue_.front());
+    write_queue_.pop_front();
+  }
+
+  write_queue_.push_back(thumbnail);
+  WriteNextThumbnail();
+  return true;
+}
+
+bool ThumbnailCache::CompressThumbnailIfNecessary(
+    scoped_refptr<Thumbnail> thumbnail) {
+  if (!thumbnail->CompressionRequired())
+    return false;
+
+  if (RemoveDuplicateIdsFromQueueHelper(compression_queue_, thumbnail))
+    return true;
+
+  if (compression_queue_.size() > compression_queue_max_size_) {
+    CleanupThumbnail(compression_queue_.front());
+    compression_queue_.pop_front();
+  }
+
+  compression_queue_.push_back(thumbnail);
+  CompressNextThumbnail();
+  return true;
+}
+
+bool ThumbnailCache::RemoveDuplicateIdsFromQueueHelper(
+    ThumbnailQueue& queue,
+    scoped_refptr<Thumbnail> thumbnail) {
+  bool found = false;
+  ThumbnailQueue remove_queue;
+  for (ThumbnailQueue::iterator iter = queue.begin(); iter != queue.end();
+       iter++) {
+    if (iter->get() == thumbnail.get())
+      found = true;
+    if ((*iter)->tab_id() == thumbnail->tab_id()) {
+      remove_queue.push_back(thumbnail);
+      CleanupThumbnail(thumbnail);
+    }
+  }
+  return found;
+}
+
+void ThumbnailCache::WriteNextThumbnail() {
+  if (!write_queue_.empty()) {
+    scoped_refptr<Thumbnail> thumbnail = write_queue_.front();
+    write_queue_.pop_front();
+
+    if (!thumbnail->IsValid())
+      return;
+
+    // Make a copy of the thumbnail for write to disk.  This copy increases the
+    // ref-count of the pixels that the thumbnail is holding on to.  We do this
+    // to avoid concurrent access issues.
+    scoped_refptr<Thumbnail> task_thumbnail =
+        make_scoped_refptr(new Thumbnail(*thumbnail));
+
+    base::Closure write_task = base::Bind(&ThumbnailCache::WriteTask,
+                                          GetFilePath(task_thumbnail->tab_id()),
+                                          task_thumbnail);
+    base::Closure post_write_task = base::Bind(&ThumbnailCache::PostWriteTask,
+                                               weak_factory_.GetWeakPtr(),
+                                               task_thumbnail);
+    content::BrowserThread::PostTaskAndReply(
+        content::BrowserThread::FILE, FROM_HERE, write_task, post_write_task);
+  }
+}
+
+void ThumbnailCache::WriteTask(const base::FilePath& file_path,
+                               scoped_refptr<Thumbnail> thumbnail) {
+  DCHECK(thumbnail);
+  DCHECK(thumbnail->IsValid());
+  base::File file(file_path,
+                  base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
+  thumbnail->WriteThumbnailToFile(file);
+  file.Close();
+}
+
+void ThumbnailCache::PostWriteTask(scoped_refptr<Thumbnail> thumbnail) {
+  DCHECK(thumbnail);
+  // If the write has failed, then this thumbnail is invalidated, we delete the
+  // associated file.  The tab Id should still be valid.
+  TabId tab_id = thumbnail->tab_id();
+  if (!thumbnail->IsValid())
+    RemoveCorruptThumbnailSource(tab_id);
+  CleanupThumbnail(GetThumbnail(tab_id));
+  WriteNextThumbnail();
+}
+
+void ThumbnailCache::ReadNextThumbnail(TabId id) {
+  read_queue_.clear();
+  if (std::find(read_queue_.begin(), read_queue_.end(), id) ==
+      read_queue_.end())
+    read_queue_.push_back(id);
+  ReadNextThumbnail();
+}
+
+void ThumbnailCache::ReadNextThumbnail() {
+  if (!read_queue_.empty()) {
+    TabId tab_id = read_queue_.front();
+    // Create a thumbnail to hold the content of the read.
+    scoped_refptr<Thumbnail> thumbnail =
+        make_scoped_refptr(new Thumbnail(tab_id, ui_resource_provider_));
+
+    base::Closure read_task = base::Bind(
+        &ThumbnailCache::ReadTask, GetFilePath(thumbnail->tab_id()), thumbnail);
+    base::Closure post_read_task = base::Bind(
+        &ThumbnailCache::PostReadTask, weak_factory_.GetWeakPtr(), thumbnail);
+    content::BrowserThread::PostTaskAndReply(
+        content::BrowserThread::FILE, FROM_HERE, read_task, post_read_task);
+  }
+}
+
+void ThumbnailCache::ReadTask(const base::FilePath& file_path,
+                              scoped_refptr<Thumbnail> thumbnail) {
+  DCHECK(thumbnail);
+  if (!base::PathExists(file_path))
+    return;
+
+  base::File file(file_path, base::File::FLAG_OPEN | base::File::FLAG_READ);
+  DCHECK(file.IsValid());
+  thumbnail->ReadThumbnailFromFile(file);
+  file.Close();
+}
+
+void ThumbnailCache::PostReadTask(scoped_refptr<Thumbnail> thumbnail) {
+  DCHECK(thumbnail);
+  TabIdList::iterator iter =
+      std::find(read_queue_.begin(), read_queue_.end(), thumbnail->tab_id());
+  if (iter == read_queue_.end())
+    return;
+
+  read_queue_.erase(iter);
+
+  // If the read has failed, then this thumbnail is invalidated, we delete the
+  // associated file.  The tab Id should still be valid.
+  TabId tab_id = thumbnail->tab_id();
+  if (!thumbnail->IsValid()) {
+    RemoveCorruptThumbnailSource(tab_id);
+    ReadNextThumbnail();
+    return;
+  }
+
+  // TODO(powei): We should be able to remove this.  All on-disk thumbnails
+  // are compressed and so are not amenable to approximation.
+  scoped_refptr<Thumbnail> approx_thumbnail;
+  if (!approximation_cache_.Contains(tab_id)) {
+    SkBitmap raw_data = thumbnail->raw_data();
+    if (!raw_data.empty() && use_approximation_thumbnails_) {
+      ThumbnailBitmap thumbnail_bitmap(raw_data, thumbnail->scale());
+      ThumbnailBitmap approx_thumbnail_bitmap =
+          thumbnail_bitmap.CreateApproximation();
+      if (approx_thumbnail_bitmap.IsValid()) {
+        approx_thumbnail = make_scoped_refptr(
+            new ApproxThumbnail(tab_id,
+                                approx_thumbnail_bitmap.bitmap(),
+                                approx_thumbnail_bitmap.scale(),
+                                ui_resource_provider_));
+      }
+    }
+  }
+  MakeSpaceForNewItemIfNecessary(tab_id);
+  scoped_refptr<Thumbnail> old_thumbnail = cache_.Remove(tab_id);
+  CleanupThumbnail(old_thumbnail);
+  cache_.Put(tab_id, thumbnail);
+  if (approx_thumbnail) {
+    scoped_refptr<Thumbnail> old_approx_thumbnail =
+        approximation_cache_.Remove(tab_id);
+    CleanupThumbnail(old_approx_thumbnail);
+    approximation_cache_.Put(tab_id, approx_thumbnail);
+  }
+
+  RebuildThumbnail(thumbnail, false);
+  NotifyListenersOfThumbnailChange(tab_id, true);
+
+  // TODO(powei): We should be able to remove this.  All on-disk thumbnails are
+  // compressed.
+  if (thumbnail->CompressionRequired()) {
+    compression_queue_.push_back(thumbnail);
+    CompressNextThumbnail();
+  }
+
+  ReadNextThumbnail();
+}
+
+void ThumbnailCache::CompressNextThumbnail() {
+  if (!compression_queue_.empty()) {
+    scoped_refptr<Thumbnail> thumbnail = compression_queue_.front();
+    compression_queue_.pop_front();
+
+    // Make a copy of the thumbnail for compression.  This copy increases the
+    // ref-count of the pixels that the thumbnail is holding on to.  We do this
+    // to avoid concurrent access issues.
+    scoped_refptr<Thumbnail> task_thumbnail =
+        make_scoped_refptr(new Thumbnail(*thumbnail));
+
+    base::Closure compression_task =
+        base::Bind(&ThumbnailCache::CompressionTask, task_thumbnail);
+    base::Closure post_compression_task =
+        base::Bind(&ThumbnailCache::PostCompressionTask,
+                   weak_factory_.GetWeakPtr(),
+                   task_thumbnail);
+
+    DCHECK(compression_thread_.message_loop_proxy());
+    compression_thread_.message_loop_proxy()->PostTaskAndReply(
+        FROM_HERE, compression_task, post_compression_task);
+  }
+}
+
+void ThumbnailCache::CompressionTask(scoped_refptr<Thumbnail> thumbnail) {
+  thumbnail->Compress();
+}
+
+void ThumbnailCache::PostCompressionTask(
+    scoped_refptr<Thumbnail> compressed_thumbnail) {
+  DCHECK(compressed_thumbnail);
+  TabId tab_id = compressed_thumbnail->tab_id();
+  if (!compressed_thumbnail->HasCompressedRawData()) {
+    // Compression has failed and we clean up.
+    CleanupThumbnail(cache_.Remove(tab_id));
+    CleanupThumbnail(compressed_thumbnail);
+    NotifyListenersOfThumbnailChange(tab_id, false);
+  } else {
+    // The thumbnail in |cache_| is not the same reference as
+    // |compressed_thumbnail|. Replace the one in |cache_|.
+    bool rebuild_thumbnail = false;
+    if (cache_.Get(tab_id)) {
+      cache_.Put(tab_id, compressed_thumbnail);
+      rebuild_thumbnail = true;
+    }
+
+    if (rebuild_thumbnail)
+      RebuildThumbnail(compressed_thumbnail, false);
+
+    WriteThumbnailIfNecessary(compressed_thumbnail);
+    CleanupThumbnail(compressed_thumbnail);
+    NotifyListenersOfThumbnailChange(tab_id, true);
+  }
+  CompressNextThumbnail();
+}
+
+void ThumbnailCache::RebuildThumbnail(scoped_refptr<Thumbnail> thumbnail,
+                                      bool force_cleanup_cpu_data) {
+  if (!thumbnail)
+    return;
+
+  thumbnail->RebuildThumbnail();
+  thumbnail->CleanupCPUData(force_cleanup_cpu_data);
+}
+
+bool ThumbnailCache::ShouldCleanupThumbnail(
+    scoped_refptr<Thumbnail> thumbnail,
+    const ExpiringThumbnailCache* forced_source) {
+  scoped_refptr<Thumbnail> cache_entry = cache_.Get(thumbnail->tab_id());
+  scoped_refptr<Thumbnail> approx_entry =
+      approximation_cache_.Get(thumbnail->tab_id());
+  return thumbnail->is_approximation() ||
+         ((cache_entry.get() != thumbnail.get() || forced_source == &cache_) &&
+          (approx_entry.get() != thumbnail.get() ||
+           forced_source == &approximation_cache_) &&
+          std::find(write_queue_.begin(), write_queue_.end(), thumbnail) ==
+              write_queue_.end() &&
+          std::find(compression_queue_.begin(),
+                    compression_queue_.end(),
+                    thumbnail) == compression_queue_.end());
+}
+
+void ThumbnailCache::CleanupThumbnail(scoped_refptr<Thumbnail> thumbnail) {
+  CleanupThumbnail(thumbnail, NULL);
+}
+
+void ThumbnailCache::CleanupThumbnail(
+    scoped_refptr<Thumbnail> thumbnail,
+    const ExpiringThumbnailCache* forced_source) {
+  if (!thumbnail)
+    return;
+
+  if (ShouldCleanupThumbnail(thumbnail, forced_source)) {
+    thumbnail->CleanupThumbnail();
+    thumbnail->CleanupCPUData(false);
+  }
+}
+
+void ThumbnailCache::NotifyListenersOfThumbnailChange(TabId tab_id,
+                                                      bool upgrade) {
+  NotifyListenersOfUIResourceUpdate(tab_id);
+
+  for (ThumbnailChangeListenerSet::iterator iter =
+           thumbnail_change_listeners_.begin();
+       iter != thumbnail_change_listeners_.end();
+       iter++) {
+    (*iter)->OnThumbnailChanged(tab_id, upgrade);
+  }
+}
+
+void ThumbnailCache::NotifyListenersOfUIResourceUpdate(TabId tab_id) {
+  scoped_refptr<Thumbnail> thumbnail = GetThumbnail(tab_id);
+  for (ThumbnailChangeListenerSet::iterator iter =
+           thumbnail_change_listeners_.begin();
+       iter != thumbnail_change_listeners_.end();
+       iter++) {
+    if (thumbnail) {
+      (*iter)->OnUIResourceUpdated(
+          tab_id,
+          thumbnail->ui_resource_id(),
+          thumbnail->GetContentSize(),
+          gfx::ScaleSize(thumbnail->data_size(), 1.f / thumbnail->scale()));
+    } else {
+      (*iter)->OnUIResourceUpdated(tab_id, 0, gfx::Size(), gfx::SizeF());
+    }
+  }
+}
+
+base::FilePath ThumbnailCache::GetFilePath(TabId tab_id) const {
+  return disk_cache_path_.Append(base::IntToString(tab_id));
+}
+
+bool ThumbnailCache::ThumbnailInputFileExists(TabId tab_id) const {
+  return base::PathExists(GetFilePath(tab_id));
+}
+
+void ThumbnailCache::RemoveCorruptThumbnailSource(TabId tab_id) {
+  RemoveFromDisk(tab_id);
+}
+
+void ThumbnailCache::MakeSpaceForNewItemIfNecessary(TabId tab_id) {
+  if (cache_.Get(tab_id) ||
+      std::find(visible_ids_.begin(), visible_ids_.end(), tab_id) ==
+          visible_ids_.end() ||
+      cache_.size() < cache_.MaximumCacheSize()) {
+    return;
+  }
+
+  RemoveThumbnailFromCache();
+}
+
+void ThumbnailCache::RemoveThumbnailFromCache() {
+  TabId key_to_remove;
+  bool found_key_to_remove = false;
+
+  // 1. Find a cached item not in this list
+  for (ExpiringThumbnailCache::iterator iter = cache_.begin();
+       iter != cache_.end();
+       iter++) {
+    if (std::find(visible_ids_.begin(), visible_ids_.end(), iter->first) ==
+        visible_ids_.end()) {
+      key_to_remove = iter->first;
+      found_key_to_remove = true;
+      break;
+    }
+  }
+
+  if (!found_key_to_remove) {
+    // 2. Find the least important id we can remove.
+    for (TabIdList::reverse_iterator riter = visible_ids_.rbegin();
+         riter != visible_ids_.rend();
+         riter++) {
+      if (cache_.Get(*riter)) {
+        key_to_remove = *riter;
+        break;
+        found_key_to_remove = true;
+      }
+    }
+  }
+
+  if (found_key_to_remove) {
+    CleanupThumbnail(cache_.Remove(key_to_remove));
+    NotifyListenersOfThumbnailChange(key_to_remove, false);
+  }
+}
+
+void ThumbnailCache::AddIdToVisibleIds(TabId tab_id) {
+  if (std::find(visible_ids_.begin(), visible_ids_.end(), tab_id) !=
+      visible_ids_.end())
+    return;
+
+  if (visible_ids_.size() > cache_.MaximumCacheSize() - 1)
+    visible_ids_.pop_back();
+
+  visible_ids_.push_front(tab_id);
+}
+
+ThumbnailCache::ThumbnailMetaData::ThumbnailMetaData() {
+}
+
+ThumbnailCache::ThumbnailMetaData::ThumbnailMetaData(
+    const base::Time& current_time,
+    const GURL& url)
+    : capture_time_(current_time), url_(url) {
+}
+
+void ThumbnailCache::RemoveFromDiskAtAndAboveId(TabId min_id) {
+  base::Closure remove_task =
+      base::Bind(&ThumbnailCache::RemoveFromDiskAtAndAboveIdTask,
+                 disk_cache_path_,
+                 min_id);
+  content::BrowserThread::PostTask(
+      content::BrowserThread::FILE, FROM_HERE, remove_task);
+}
+
+void ThumbnailCache::RemoveFromDisk(TabId tab_id) {
+  base::Closure remove_task =
+      base::Bind(&ThumbnailCache::RemoveFromDiskTask, GetFilePath(tab_id));
+  content::BrowserThread::PostTask(
+      content::BrowserThread::FILE, FROM_HERE, remove_task);
+}
+
+void ThumbnailCache::RemoveFromDiskTask(const base::FilePath& file_path) {
+  base::DeleteFile(file_path, false);
+}
+
+void ThumbnailCache::RemoveFromDiskAtAndAboveIdTask(
+    const base::FilePath& dir_path,
+    TabId min_id) {
+  base::FileEnumerator enumerator(dir_path, false, base::FileEnumerator::FILES);
+  while (true) {
+    base::FilePath path = enumerator.Next();
+    if (path.empty())
+      break;
+    base::FileEnumerator::FileInfo info = enumerator.GetInfo();
+    TabId tab_id;
+    bool success = base::StringToInt(info.GetName().value(), &tab_id);
+    if (success && tab_id >= min_id)
+      base::DeleteFile(path, false);
+  }
+}
+
+void ThumbnailCache::ThumbnailRequested(TabId tab_id) {
+  scoped_refptr<Thumbnail> thumbnail = GetThumbnail(tab_id);
+  if (thumbnail) {
+    if (!thumbnail->RebuildThumbnail())
+      return;
+    cc::UIResourceId ui_resource_id = thumbnail->ui_resource_id();
+    if (!ui_resource_id)
+      return;
+    NotifyListenersOfUIResourceUpdate(tab_id);
+  }
+}
+
+void ThumbnailCache::OnUIResourcesAreInvalid() {
+  RemoveThumbnailsAndScheduleReload();
+}
+
+void ThumbnailCache::OnRecreateUIResources() {
+  RemoveThumbnailsAndScheduleReload();
+}
+
+base::Time ThumbnailCache::CurrentTime() {
+  return base::Time::Now();
+}
+
+ThumbnailCache::Thumbnail::Thumbnail(
+    TabId tab_id,
+    content::UIResourceProvider* ui_resource_provider)
+    : tab_id_(tab_id),
+      ui_resource_provider_(ui_resource_provider),
+      ui_resource_id_(0),
+      format_(DECOMPRESSED),
+      rebuild_pending_(false),
+      data_write_pending_(false),
+      from_stream_(false) {
+  DCHECK(ui_resource_provider);
+}
+
+ThumbnailCache::Thumbnail::Thumbnail(
+    TabId tab_id,
+    SkBitmap bitmap,
+    float scale,
+    content::UIResourceProvider* ui_resource_provider)
+    : tab_id_(tab_id),
+      raw_data_(bitmap),
+      scale_(scale),
+      ui_resource_provider_(ui_resource_provider),
+      ui_resource_id_(0),
+      format_(DECOMPRESSED),
+      rebuild_pending_(false),
+      data_write_pending_(false),
+      from_stream_(false) {
+  DCHECK(ui_resource_provider);
+  SetRawThumbnailData(bitmap);
+}
+
+ThumbnailCache::Thumbnail::Thumbnail(const Thumbnail& thumbnail)
+    : tab_id_(thumbnail.tab_id_),
+      raw_data_(thumbnail.raw_data_),
+      scale_(thumbnail.scale_),
+      ui_resource_provider_(thumbnail.ui_resource_provider_),
+      ui_resource_id_(thumbnail.ui_resource_id_),
+      compressed_data_(thumbnail.compressed_data_),
+      data_size_(thumbnail.data_size_),
+      scaled_content_size_(thumbnail.scaled_content_size_),
+      format_(thumbnail.format_),
+      rebuild_pending_(thumbnail.rebuild_pending_),
+      data_write_pending_(thumbnail.data_write_pending_),
+      from_stream_(thumbnail.from_stream_) {
+}
+
+ThumbnailCache::Thumbnail::~Thumbnail() {
+}
+
+ThumbnailCache::ApproxThumbnail::ApproxThumbnail(
+    TabId tab_id,
+    SkBitmap bitmap,
+    float scale,
+    content::UIResourceProvider* ui_resource_provider)
+    : Thumbnail(tab_id, bitmap, scale, ui_resource_provider) {
+}
+
+void ThumbnailCache::Thumbnail::CleanupThumbnail() {
+  if (ui_resource_id_) {
+    ui_resource_provider_->DeleteUIResource(ui_resource_id_);
+    ui_resource_id_ = 0;
+  }
+}
+
+bool ThumbnailCache::Thumbnail::AttemptToScheduleRebuildFromData() {
+  if (!raw_data_.empty() ||
+      (compressed_data_ && !compressed_data_->info().isEmpty())) {
+    rebuild_pending_ = true;
+    return true;
+  }
+  return false;
+}
+
+void ThumbnailCache::Thumbnail::CleanupCPUData(bool force) {
+  if (!CleanupRequired() && !force)
+    return;
+  raw_data_ = SkBitmap();
+  compressed_data_.clear();
+}
+
+bool ThumbnailCache::Thumbnail::RebuildThumbnail() {
+  if (!rebuild_pending_)
+    return false;
+  CleanupThumbnail();
+  if ((raw_data_.empty() && format_ == DECOMPRESSED) ||
+      (!compressed_data_ && format_ == COMPRESSED))
+    return false;
+  bool is_bitmap_transient = true;
+  if (format_ == COMPRESSED) {
+    ui_resource_id_ = ui_resource_provider_->GenerateCompressedUIResource(
+        compressed_data_, is_bitmap_transient);
+  } else {
+    ui_resource_id_ = ui_resource_provider_->GenerateUIResource(
+        raw_data_, is_bitmap_transient);
+  }
+
+  rebuild_pending_ = false;
+  CleanupCPUData(false);
+  return true;
+}
+
+bool ThumbnailCache::Thumbnail::CleanupRequired() {
+  return !(CompressionRequired() || DataWriteRequired() || rebuild_pending_ ||
+           (raw_data_.empty() && !compressed_data_));
+}
+
+bool ThumbnailCache::Thumbnail::CompressionRequired() const {
+  return kEnableCompression && !compressed_data_ && !raw_data_.empty();
+}
+
+bool ThumbnailCache::Thumbnail::DataWriteRequired() const {
+  return data_write_pending_ && !from_stream_;
+}
+
+bool ThumbnailCache::Thumbnail::HasCompressedRawData() const {
+  return compressed_data_;
+}
+
+bool ThumbnailCache::Thumbnail::IsValid() const {
+  return ui_resource_provider_ && scale_ > 0 && !data_size_.IsEmpty() &&
+         !scaled_content_size_.IsEmpty() &&
+         (ui_resource_id_ || !raw_data_.empty() || compressed_data_);
+}
+
+gfx::Size ThumbnailCache::Thumbnail::GetContentSize() const {
+  return gfx::ToRoundedSize(gfx::ScaleSize(scaled_content_size_, 1.f / scale_));
+}
+
+void ThumbnailCache::Thumbnail::Compress() {
+  SkBitmap raw_data = raw_data_;
+  if (CompressionRequired() && format_ == DECOMPRESSED && !raw_data.empty()) {
+    SkAutoLockPixels raw_data_lock(raw_data);
+    gfx::Size raw_data_size(raw_data.width(), raw_data.height());
+    DCHECK_EQ(raw_data.config(), SkBitmap::kARGB_8888_Config);
+    size_t pixel_size = 4;  // Pixel size is 4 bytes for kARGB_8888_Config.
+    size_t stride = pixel_size * raw_data_size.width();
+
+    gfx::Size encoded_size = GetEncodedSize(raw_data_size);
+    size_t encoded_bytes =
+        etc1_get_encoded_data_size(encoded_size.width(), encoded_size.height());
+    SkImageInfo info = {encoded_size.width(),
+                        encoded_size.height(),
+                        kUnknown_SkColorType,
+                        kUnpremul_SkAlphaType};
+    skia::RefPtr<SkData> etc1_pixel_data = skia::AdoptRef(
+        SkData::NewFromMalloc(new uint8_t[encoded_bytes], encoded_bytes));
+    skia::RefPtr<SkMallocPixelRef> etc1_pixel_ref = skia::AdoptRef(
+        SkMallocPixelRef::NewWithData(info, 0, NULL, etc1_pixel_data.get()));
+
+    etc1_pixel_ref->lockPixels();
+    bool success = etc1_encode_image(
+        reinterpret_cast<unsigned char*>(raw_data.getPixels()),
+        raw_data_size.width(),
+        raw_data_size.height(),
+        pixel_size,
+        stride,
+        reinterpret_cast<unsigned char*>(etc1_pixel_ref->pixels()),
+        encoded_size.width(),
+        encoded_size.height());
+    etc1_pixel_ref->unlockPixels();
+    if (success) {
+      etc1_pixel_ref->setImmutable();
+      SetCompressedThumbnailData(etc1_pixel_ref, raw_data_size);
+    } else {
+      Invalidate();
+    }
+  }
+}
+
+bool ThumbnailCache::Thumbnail::WriteThumbnailToFile(base::File& file) {
+  // We assume caller has already initialized the file and will also close it.
+  DCHECK(file.IsValid());
+
+  if (DataWriteRequired() && format_ == COMPRESSED && compressed_data_) {
+    if (kEnableCompression) {
+      compressed_data_->lockPixels();
+      bool success = true;
+      int scaled_content_width = scaled_content_size_.width();
+      int scaled_content_height = scaled_content_size_.height();
+      int data_width = data_size_.width();
+      int data_height = data_size_.height();
+
+      if (file.WriteAtCurrentPos(reinterpret_cast<const char*>(&kCompressedKey),
+                                 sizeof(int)) < 0 ||
+          file.WriteAtCurrentPos(
+              reinterpret_cast<const char*>(&scaled_content_width),
+              sizeof(int)) < 0 ||
+          file.WriteAtCurrentPos(
+              reinterpret_cast<const char*>(&scaled_content_height),
+              sizeof(int)) < 0 ||
+          file.WriteAtCurrentPos(reinterpret_cast<const char*>(&data_width),
+                                 sizeof(int)) < 0 ||
+          file.WriteAtCurrentPos(reinterpret_cast<const char*>(&data_height),
+                                 sizeof(int)) < 0 ||
+          file.WriteAtCurrentPos(reinterpret_cast<const char*>(&scale_),
+                                 sizeof(float)) < 0) {
+        success = false;
+      }
+
+      size_t compressed_bytes =
+          etc1_get_encoded_data_size(data_size_.width(), data_size_.height());
+
+      if (file.WriteAtCurrentPos(
+              reinterpret_cast<char*>(compressed_data_->pixels()),
+              compressed_bytes) < 0)
+        success = false;
+
+      compressed_data_->unlockPixels();
+
+      if (!success)
+        Invalidate();
+
+      return success;
+    } else {
+      // In the original Java implementation, we store a jpg version when the
+      // |kEnableCompression| flag is off.  Keep as NOTIMPLEMENTED for now.
+      NOTIMPLEMENTED();
+    }
+  }
+  return false;
+}
+
+bool ThumbnailCache::Thumbnail::ReadThumbnailFromFile(base::File& file) {
+  // We assume caller has already initialized the file and will also close it.
+  DCHECK(file.IsValid());
+
+  if (kEnableCompression) {
+    int key;
+    bool success = true;
+    if (file.ReadAtCurrentPos(reinterpret_cast<char*>(&key), sizeof(int)) < 0 ||
+        key != kCompressedKey)
+      success = false;
+
+    int width, height;
+    if (file.ReadAtCurrentPos(reinterpret_cast<char*>(&width), sizeof(int)) <
+            0 ||
+        file.ReadAtCurrentPos(reinterpret_cast<char*>(&height), sizeof(int)) <
+            0)
+      success = false;
+    scaled_content_size_ = gfx::Size(width, height);
+
+    if (file.ReadAtCurrentPos(reinterpret_cast<char*>(&width), sizeof(int)) <
+            0 ||
+        file.ReadAtCurrentPos(reinterpret_cast<char*>(&height), sizeof(int)) <
+            0)
+      success = false;
+    data_size_ = gfx::Size(width, height);
+
+    if (file.ReadAtCurrentPos(reinterpret_cast<char*>(&scale_), sizeof(float)) <
+        0)
+      success = false;
+
+    size_t compressed_bytes =
+        etc1_get_encoded_data_size(data_size_.width(), data_size_.height());
+
+    SkImageInfo info = {data_size_.width(),
+                        data_size_.height(),
+                        kUnknown_SkColorType,
+                        kUnpremul_SkAlphaType};
+
+    scoped_ptr<uint8_t[]> data(new uint8_t[compressed_bytes]);
+    if (file.ReadAtCurrentPos(reinterpret_cast<char*>(data.get()),
+                              compressed_bytes) < 0)
+      success = false;
+
+    skia::RefPtr<SkData> etc1_pixel_data =
+        skia::AdoptRef(SkData::NewFromMalloc(data.release(), compressed_bytes));
+    compressed_data_ = skia::AdoptRef(
+        SkMallocPixelRef::NewWithData(info, 0, NULL, etc1_pixel_data.get()));
+
+    format_ = COMPRESSED;
+    from_stream_ = true;
+    rebuild_pending_ = true;
+    data_write_pending_ = false;
+
+    if (!success)
+      Invalidate();
+
+    return success;
+  } else {
+    NOTIMPLEMENTED();
+  }
+
+  return false;
+}
+
+void ThumbnailCache::Thumbnail::Invalidate() {
+  CleanupCPUData(true);
+  ui_resource_id_ = 0;
+  data_size_ = gfx::Size();
+  scaled_content_size_ = gfx::Size();
+}
+
+void ThumbnailCache::Thumbnail::SetCompressedThumbnailData(
+    skia::RefPtr<SkPixelRef> compressed_data,
+    gfx::Size scaled_content_size) {
+  compressed_data_ = compressed_data;
+  raw_data_ = SkBitmap();
+  if (compressed_data) {
+    data_size_ = gfx::Size(compressed_data_->info().width(),
+                           compressed_data_->info().height());
+    scaled_content_size_ = scaled_content_size;
+    format_ = COMPRESSED;
+    rebuild_pending_ = true;
+    data_write_pending_ = true;
+  } else {
+    data_size_ = gfx::Size();
+    scaled_content_size_ = gfx::Size();
+    rebuild_pending_ = false;
+    data_write_pending_ = false;
+  }
+}
+
+void ThumbnailCache::Thumbnail::SetRawThumbnailData(SkBitmap bitmap) {
+  raw_data_ = bitmap;
+  compressed_data_.clear();
+  data_size_ = gfx::Size(bitmap.width(), bitmap.height());
+  scaled_content_size_ = data_size_;
+  format_ = DECOMPRESSED;
+  rebuild_pending_ = true;
+  data_write_pending_ = !kEnableCompression;
+}
+
+ThumbnailCache::ThumbnailBitmap::ThumbnailBitmap() : scale_(0.f) {
+}
+
+ThumbnailCache::ThumbnailBitmap::ThumbnailBitmap(SkBitmap bitmap, float scale)
+    : bitmap_(bitmap), scale_(scale) {
+}
+
+bool ThumbnailCache::ThumbnailBitmap::IsValid() const {
+  return !bitmap_.empty() && scale_ > 0;
+}
+
+ThumbnailCache::ThumbnailBitmap
+ThumbnailCache::ThumbnailBitmap::CreateApproximation() const {
+  SkAutoLockPixels bitmap_lock(bitmap_);
+  float new_scale = 1.f / kApproximationScaleFactor;
+
+  gfx::Size dst_size = gfx::ToFlooredSize(
+      gfx::ScaleSize(gfx::Size(bitmap_.width(), bitmap_.height()), new_scale));
+  SkBitmap dst_bitmap;
+  dst_bitmap.setConfig(
+      bitmap_.getConfig(), dst_size.width(), dst_size.height());
+  dst_bitmap.allocPixels();
+  dst_bitmap.eraseColor(0);
+  SkAutoLockPixels dst_bitmap_lock(dst_bitmap);
+
+  SkCanvas canvas(dst_bitmap);
+  canvas.scale(new_scale, new_scale);
+  canvas.drawBitmap(bitmap_, 0, 0, NULL);
+  dst_bitmap.setImmutable();
+
+  return ThumbnailBitmap(dst_bitmap, new_scale * scale_);
+}
+
+ThumbnailCache::TabReadbackRequest::TabReadbackRequest(
+    TabId tab_id,
+    content::ContentViewCore* content_view_core,
+    float thumbnail_scale,
+    base::Callback<void(int, const ThumbnailBitmap&)> end_callback)
+    : tab_id_(tab_id),
+      content_view_core_(content_view_core),
+      thumbnail_scale_(thumbnail_scale),
+      end_callback_(end_callback),
+      drop_after_readback_(false),
+      weak_factory_(this) {
+}
+
+void ThumbnailCache::TabReadbackRequest::Run() {
+  base::Callback<void(bool, const SkBitmap&)> result_callback =
+      base::Bind(&TabReadbackRequest::OnFinishGetTabThumbnailBitmap,
+                 weak_factory_.GetWeakPtr());
+  if (content_view_core_) {
+    DCHECK(content_view_core_->GetWebContents());
+    content_view_core_->GetWebContents()
+        ->GetRenderViewHost()
+        ->LockBackingStore();
+
+    // Calling this method with an empty rect will return a bitmap of the size
+    // of the content.
+    content_view_core_->GetScaledContentBitmap(thumbnail_scale_,
+                                               SkBitmap::kARGB_8888_Config,
+                                               gfx::Rect(),
+                                               result_callback);
+  } else {
+    result_callback.Run(false, SkBitmap());
+  }
+}
+
+// This callback should be called on the UI thread.  So it should be safe to
+// call the thumbnail cache.
+void ThumbnailCache::TabReadbackRequest::OnFinishGetTabThumbnailBitmap(
+    bool success,
+    const SkBitmap& bitmap) {
+  if (content_view_core_) {
+    DCHECK(content_view_core_->GetWebContents());
+    content_view_core_->GetWebContents()
+        ->GetRenderViewHost()
+        ->UnlockBackingStore();
+  }
+  if (success) {
+    SkBitmap local_bitmap = bitmap;
+    local_bitmap.setImmutable();
+    end_callback_.Run(tab_id_, ThumbnailBitmap(local_bitmap, thumbnail_scale_));
+  } else {
+    end_callback_.Run(tab_id_, ThumbnailBitmap());
+  }
+}