Fix login visual hitch on chromebook

chrome/browser/chromeos/extensions/wallpaper_private_api.cc

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 2013 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/chromeos/extensions/wallpaper_private_api.h"
 
 #include <vector>
 
 #include "ash/shell.h"
 #include "ash/wm/window_cycle_controller.h"
 #include "ash/wm/window_util.h"
@@ skipping 211 matching lines @@
   return true;
 }
 
 class WallpaperFunctionBase::WallpaperDecoder : public ImageDecoder::Delegate {
  public:
   explicit WallpaperDecoder(scoped_refptr<WallpaperFunctionBase> function)
       : function_(function) {
   }
 
   void Start(const std::string& image_data) {
+    DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
     image_decoder_ = new ImageDecoder(this, image_data,
                                       ImageDecoder::ROBUST_JPEG_CODEC);
-    image_decoder_->Start();
+    scoped_refptr<base::MessageLoopProxy> task_runner =
+        BrowserThread::GetMessageLoopProxyForThread(BrowserThread::UI);
+    image_decoder_->Start(task_runner);
   }
 
   void Cancel() {
     cancel_flag_.Set();
   }
 
   virtual void OnImageDecoded(const ImageDecoder* decoder,
                               const SkBitmap& decoded_image) OVERRIDE {
     gfx::ImageSkia final_image(decoded_image);
     final_image.MakeThreadSafe();
@@ skipping 445 matching lines @@
                  this, file_list));
 }
 
 void WallpaperGetOfflineWallpaperListFunction::OnComplete(
     const std::vector<std::string>& file_list) {
   ListValue* results = new ListValue();
   results->AppendStrings(file_list);
   SetResult(results);
   SendResponse(true);
 }