Move the IconLoader to use the task scheduler.

chrome/browser/icon_loader.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_ICON_LOADER_H_
 #define CHROME_BROWSER_ICON_LOADER_H_
 
 #include <memory>
 #include <string>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/single_thread_task_runner.h"
+#include "base/task_scheduler/task_traits.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/gfx/image/image.h"
 
 ////////////////////////////////////////////////////////////////////////////////
 //
 // A facility to read a file containing an icon asynchronously in the IO
 // thread. Returns the icon in the form of an ImageSkia.
 //
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 34 matching lines @@
  private:
   IconLoader(const base::FilePath& file_path,
              IconSize size,
              IconLoadedCallback callback);
 
   ~IconLoader();
 
   // Given a file path, get the group for the given file.
   static IconGroup GroupForFilepath(const base::FilePath& file_path);
 
-  // The thread ReadIcon() should be called on.
-  static content::BrowserThread::ID ReadIconThreadID();
+  // The TaskRunner that ReadIcon() must be called on.
+  static scoped_refptr<base::TaskRunner> ReadIconTaskRunner();

[sky, 2017/06/22 21:20:49]

Mild preference for GetReadIconTaskRunner() as read-icon-task-runner sounds like
a verb.

[Avi (use Gerrit), 2017/06/22 21:46:34]

Done.

 
   void ReadGroup();
-  void OnReadGroup();
   void ReadIcon();
 
+  // The traits of the tasks posted by this class. These operations may block,
+  // because they are fetching icons from the disk, yet the result will be seen
+  // by the user so they should be prioritized accordingly.
+  static constexpr base::TaskTraits traits() {
+    return {base::MayBlock(), base::TaskPriority::USER_VISIBLE};
+  }
+
   // The task runner object of the thread in which we notify the delegate.
   scoped_refptr<base::SingleThreadTaskRunner> target_task_runner_;
 
   base::FilePath file_path_;
 
   IconGroup group_;
 
   IconSize icon_size_;
 
   IconLoadedCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(IconLoader);
 };
 
 #endif  // CHROME_BROWSER_ICON_LOADER_H_