Pass the ImageSkia by pointer instead of by reference, which will create shallow copy.

chrome/browser/chromeos/login/wallpaper_manager.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_LOGIN_WALLPAPER_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_WALLPAPER_MANAGER_H_
 
 #include <string>
 
 #include "ash/desktop_background/desktop_background_resources.h"
@@ skipping 188 matching lines @@
   // For case 2, should_cache_wallpaper_ is used to indicate if we need to
   // cache wallpapers on wallpaper animation finished. The cache operation
   // should be only executed once.
   void CacheAllUsersWallpapers();
 
   // Caches |email|'s wallpaper to memory.
   void CacheUserWallpaper(const std::string& email);
 
   // Generates a 128x80 thumbnail and caches it.
   void CacheThumbnail(const std::string& email,
-                      const gfx::ImageSkia& wallpaper);
+                      scoped_ptr<gfx::ImageSkia> wallpaper);
 
   // Deletes a list of wallpaper files in |file_list|.
   void DeleteWallpaperInList(const std::vector<FilePath>& file_list);
 
   // Deletes all |email| related custom or converted wallpapers.
   void DeleteUserWallpapers(const std::string& email);
 
   // Loads |email|'s wallpaper. When |update_wallpaper| is true, sets wallpaper
   // to the loaded wallpaper.
   void LoadWallpaper(const std::string& email,
@@ skipping 114 matching lines @@
   content::NotificationRegistrar registrar_;
 
   base::OneShotTimer<WallpaperManager> timer_;
 
   DISALLOW_COPY_AND_ASSIGN(WallpaperManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_WALLPAPER_MANAGER_H_