Refactor TabContentManager to async create ThumbnailCache.

chrome/browser/android/thumbnail/thumbnail_cache.cc

 // 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/thumbnail_cache.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/big_endian.h"
@@ skipping 109 matching lines @@
     : disk_cache_path_(disk_cache_path_str),
       compression_queue_max_size_(compression_queue_max_size),
       write_queue_max_size_(write_queue_max_size),
       use_approximation_thumbnail_(use_approximation_thumbnail),
       compression_tasks_count_(0),
       write_tasks_count_(0),
       read_in_progress_(false),
       cache_(default_cache_size),
       approximation_cache_(approximation_cache_size),
       ui_resource_provider_(NULL),
-      weak_factory_(this) {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
-}
+      weak_factory_(this) {}
 
 ThumbnailCache::~ThumbnailCache() {
   SetUIResourceProvider(NULL);
 }
 
 void ThumbnailCache::SetUIResourceProvider(
     ui::UIResourceProvider* ui_resource_provider) {
   if (ui_resource_provider_ == ui_resource_provider)
     return;
 
@@ skipping 778 matching lines @@
   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 std::make_pair(dst_bitmap, new_scale * scale);
 }