Fix the crash in DesktopBackgroundView::OnPaint

chrome/browser/chromeos/extensions/wallpaper_private_api.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_CHROMEOS_EXTENSIONS_WALLPAPER_PRIVATE_API_H_
 #define CHROME_BROWSER_CHROMEOS_EXTENSIONS_WALLPAPER_PRIVATE_API_H_
 
 #include "ash/desktop_background/desktop_background_resources.h"
 #include "chrome/browser/extensions/extension_function.h"
 #include "net/url_request/url_fetcher_delegate.h"
@@ skipping 45 matching lines @@
   // AsyncExtensionFunction overrides.
   virtual bool RunImpl() OVERRIDE;
 
  private:
   virtual void OnWallpaperDecoded(const gfx::ImageSkia& wallpaper) OVERRIDE;
 
   // Saves the image data to a file.
   void SaveToFile();
 
   // Sets wallpaper to the decoded image.
-  void SetDecodedWallpaper();
+  void SetDecodedWallpaper(scoped_ptr<gfx::ImageSkia> wallpaper);
 
   // Layout of the downloaded wallpaper.
   ash::WallpaperLayout layout_;
 
-  // The decoded wallpaper.
+  // The decoded wallpaper. It may accessed from UI thread to set wallpaper or
+  // FILE thread to resize and save wallpaper to disk.
   gfx::ImageSkia wallpaper_;
 
   // Email address of logged in user.
   std::string email_;
 
   // High resolution wallpaper URL.
   std::string url_;
 
   // String representation of downloaded wallpaper.
   std::string image_data_;
@@ skipping 44 matching lines @@
   WallpaperRestoreMinimizedWindowsFunction();
 
  protected:
   virtual ~WallpaperRestoreMinimizedWindowsFunction();
 
   // AsyncExtensionFunction overrides.
   virtual bool RunImpl() OVERRIDE;
 };
 
 #endif  // CHROME_BROWSER_CHROMEOS_EXTENSIONS_WALLPAPER_PRIVATE_API_H_