Move the IconLoader to use the task scheduler.

chrome/browser/icon_loader_mac.mm

 // 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.
 
 #include "chrome/browser/icon_loader.h"
 
 #import <AppKit/AppKit.h>
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/threading/thread.h"
 #include "ui/gfx/image/image_skia.h"
 #include "ui/gfx/image/image_skia_util_mac.h"
 
 // static
 IconLoader::IconGroup IconLoader::GroupForFilepath(
     const base::FilePath& file_path) {
   return file_path.Extension();
 }
 
 // static
-content::BrowserThread::ID IconLoader::ReadIconThreadID() {
-  return content::BrowserThread::FILE;
+bool IconLoader::ReadIconRequiresUIThread() {
+  return false;
 }
 
 void IconLoader::ReadIcon() {
   NSString* group = base::SysUTF8ToNSString(group_);
   NSWorkspace* workspace = [NSWorkspace sharedWorkspace];
   NSImage* icon = [workspace iconForFileType:group];
 
   std::unique_ptr<gfx::Image> image;
 
   if (icon_size_ == ALL) {
@@ skipping 15 matching lines @@
     if (!image_skia.isNull()) {
       image_skia.MakeThreadSafe();
       image = base::MakeUnique<gfx::Image>(image_skia);
     }
   }
 
   target_task_runner_->PostTask(
       FROM_HERE, base::Bind(callback_, base::Passed(&image), group_));
   delete this;
 }