Switch BookmarkFaviconFetcher to the TaskScheduler API.

chrome/browser/bookmarks/bookmark_html_writer.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_BOOKMARKS_BOOKMARK_HTML_WRITER_H_
 #define CHROME_BROWSER_BOOKMARKS_BOOKMARK_HTML_WRITER_H_
 
 #include <list>
 #include <map>
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/task/cancelable_task_tracker.h"
+#include "base/task_scheduler/post_task.h"
 #include "components/favicon_base/favicon_types.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
 class Profile;
 
 namespace bookmarks {
 class BookmarkNode;
 }
 
@@ skipping 39 matching lines @@
 
   // Executes Writer task that writes bookmarks data to html file.
   void ExecuteWriter();
 
   // Starts async fetch for the next bookmark favicon.
   // Takes single url from bookmark_urls_ and removes it from the list.
   // Returns true if there are more favicons to extract.
   bool FetchNextFavicon();
 
   // Favicon fetch callback. After all favicons are fetched executes
-  // html output on the file thread.
+  // html output with |background_io_task_runner_|.
   void OnFaviconDataAvailable(
       const favicon_base::FaviconRawBitmapResult& bitmap_result);
 
   // The Profile object used for accessing FaviconService, bookmarks model.
   Profile* profile_;
 
   // All URLs that are extracted from bookmarks. Used to fetch favicons
   // for each of them. After favicon is fetched top url is removed from list.
   std::list<std::string> bookmark_urls_;
 
   // Tracks favicon tasks.
   base::CancelableTaskTracker cancelable_task_tracker_;
 
   // Map that stores favicon per URL.
   std::unique_ptr<URLFaviconMap> favicons_map_;
 
   // Path where html output is stored.
   base::FilePath path_;
 
   BookmarksExportObserver* observer_;
 
   content::NotificationRegistrar registrar_;
 
+  scoped_refptr<base::SequencedTaskRunner> background_io_task_runner_ =
+      base::CreateSequencedTaskRunnerWithTraits(
+          {base::MayBlock(), base::TaskPriority::BACKGROUND});
+
   DISALLOW_COPY_AND_ASSIGN(BookmarkFaviconFetcher);
 };
 
 namespace bookmark_html_writer {
 
 // Writes the bookmarks out in the 'bookmarks.html' format understood by
-// Firefox and IE. The results are written to the file at |path|.  The file
-// thread is used.
+// Firefox and IE. The results are written asynchronously to the file at |path|.
 // Before writing to the file favicons are fetched on the main thread.
 // TODO(sky): need a callback on failure.
 void WriteBookmarks(Profile* profile,
                     const base::FilePath& path,
                     BookmarksExportObserver* observer);
 
 }  // namespace bookmark_html_writer
 
 #endif  // CHROME_BROWSER_BOOKMARKS_BOOKMARK_HTML_WRITER_H_