Revert "[MD Settings] Remove jank for webui."

content/browser/webui/url_data_manager_backend.cc

Index: content/browser/webui/url_data_manager_backend.cc
diff --git a/content/browser/webui/url_data_manager_backend.cc b/content/browser/webui/url_data_manager_backend.cc
index 8a3e910d319ab5e6e23b918aa0c8b835c9ba27e0..404e834ca0d7542a3723e37baa9e0370a4958f98 100644
--- a/content/browser/webui/url_data_manager_backend.cc
+++ b/content/browser/webui/url_data_manager_backend.cc
@@ -22,7 +22,6 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
-#include "base/threading/worker_pool.h"
 #include "base/trace_event/trace_event.h"
 #include "content/browser/blob_storage/chrome_blob_storage_context.h"
 #include "content/browser/histogram_internals_request_job.h"
@@ -105,20 +104,6 @@ std::string GetOriginHeaderValue(const net::URLRequest* request) {
   return result;
 }
 
-// Copy data from source buffer into IO buffer destination.
-// TODO(groby): Very similar to URLRequestSimpleJob, unify at some point.
-void CopyData(const scoped_refptr<net::IOBuffer>& buf,
-              int buf_size,
-              const scoped_refptr<base::RefCountedMemory>& data,
-              int64_t data_offset) {
-  // TODO(pkasting): Remove ScopedTracker below once crbug.com/455423 is
-  // fixed.
-  tracked_objects::ScopedTracker tracking_profile(
-      FROM_HERE_WITH_EXPLICIT_FUNCTION(
-          "455423 URLRequestChromeJob::CompleteRead memcpy"));
-  memcpy(buf->data(), data->front() + data_offset, buf_size);
-}
-
 }  // namespace
 
 // URLRequestChromeJob is a net::URLRequestJob that manages running
@@ -219,9 +204,10 @@ class URLRequestChromeJob : public net::URLRequestJob {
   static void DelayStartForDevTools(
       const base::WeakPtr<URLRequestChromeJob>& job);
 
-  // Post a task to copy |data_| to |buf_| on a worker thread, to avoid browser
-  // jank. (|data_| might be mem-mapped, so a memcpy can trigger file ops).
-  int PostReadTask(net::IOBuffer* buf, int buf_size);
+  // Do the actual copy from data_ (the data we're serving) into |buf|.
+  // Separate from ReadRawData so we can handle async I/O. Returns the number of
+  // bytes read.
+  int CompleteRead(net::IOBuffer* buf, int buf_size);
 
   // The actual data we're serving.  NULL until it's been fetched.
   scoped_refptr<base::RefCountedMemory> data_;
@@ -378,22 +364,17 @@ void URLRequestChromeJob::MimeTypeAvailable(const std::string& mime_type) {
 
 void URLRequestChromeJob::DataAvailable(base::RefCountedMemory* bytes) {
   TRACE_EVENT_ASYNC_END0("browser", "DataManager:Request", this);
-  DCHECK(!data_);
-
-  // A passed-in nullptr signals an error.
-  if (!bytes) {
-    ReadRawDataComplete(net::ERR_FAILED);
-    return;
-  }
-
-  // All further requests will be satisfied from the passed-in data.
-  data_ = bytes;
-
-  if (pending_buf_) {
-    int result = PostReadTask(pending_buf_.get(), pending_buf_size_);
-    if (result != net::ERR_IO_PENDING)
+  if (bytes) {
+    data_ = bytes;
+    if (pending_buf_.get()) {
+      CHECK(pending_buf_->data());
+      int result = CompleteRead(pending_buf_.get(), pending_buf_size_);
+      pending_buf_ = NULL;
       ReadRawDataComplete(result);
-    pending_buf_ = nullptr;
+    }
+  } else {
+    // The request failed.
+    ReadRawDataComplete(net::ERR_FAILED);
   }
 }
 
@@ -402,38 +383,32 @@ base::WeakPtr<URLRequestChromeJob> URLRequestChromeJob::AsWeakPtr() {
 }
 
 int URLRequestChromeJob::ReadRawData(net::IOBuffer* buf, int buf_size) {
-  DCHECK(!pending_buf_.get());
-
-  // If data isn't available yet, mark this as asynchronous.
   if (!data_.get()) {
+    DCHECK(!pending_buf_.get());
+    CHECK(buf->data());
     pending_buf_ = buf;
     pending_buf_size_ = buf_size;
     return net::ERR_IO_PENDING;
   }
 
-  return PostReadTask(buf, buf_size);
+  // Otherwise, the data is available.
+  return CompleteRead(buf, buf_size);
 }
 
-int URLRequestChromeJob::PostReadTask(net::IOBuffer* buf, int buf_size) {
-  DCHECK(buf);
-  DCHECK(data_);
-  CHECK(buf->data());
-
+int URLRequestChromeJob::CompleteRead(net::IOBuffer* buf, int buf_size) {
   int remaining = data_->size() - data_offset_;
   if (buf_size > remaining)
     buf_size = remaining;
-
-  if (buf_size == 0)
-    return 0;
-
-  base::WorkerPool::GetTaskRunner(false)->PostTaskAndReply(
-      FROM_HERE, base::Bind(&CopyData, base::RetainedRef(buf), buf_size, data_,
-                            data_offset_),
-      base::Bind(&URLRequestChromeJob::ReadRawDataComplete, AsWeakPtr(),
-                 buf_size));
-  data_offset_ += buf_size;
-
-  return net::ERR_IO_PENDING;
+  if (buf_size > 0) {
+    // TODO(pkasting): Remove ScopedTracker below once crbug.com/455423 is
+    // fixed.
+    tracked_objects::ScopedTracker tracking_profile(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "455423 URLRequestChromeJob::CompleteRead memcpy"));
+    memcpy(buf->data(), data_->front() + data_offset_, buf_size);
+    data_offset_ += buf_size;
+  }
+  return buf_size;
 }
 
 void URLRequestChromeJob::DelayStartForDevTools(