Add some error messages for setting wallpaper failures on Chrome(c++) side.

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 19 matching lines @@
 
  protected:
   virtual ~WallpaperFunctionBase();
 
   // A class to decode JPEG file.
   class WallpaperDecoder;
 
   // Holds an instance of WallpaperDecoder.
   static WallpaperDecoder* wallpaper_decoder_;
 
+  // Handles failure or cancel cases. Passes error message to Javascript side.
+  void OnFailureOrCancel(const std::string& error);
+
  private:
   virtual void OnWallpaperDecoded(const gfx::ImageSkia& wallpaper) = 0;
-  virtual void OnFailureOrCancel() = 0;
 };
 
 class WallpaperSetWallpaperFunction : public WallpaperFunctionBase {
  public:
   DECLARE_EXTENSION_FUNCTION_NAME("wallpaperPrivate.setWallpaper");
 
   WallpaperSetWallpaperFunction();
 
  protected:
   virtual ~WallpaperSetWallpaperFunction();
 
   // AsyncExtensionFunction overrides.
   virtual bool RunImpl() OVERRIDE;
 
  private:
   virtual void OnWallpaperDecoded(const gfx::ImageSkia& wallpaper) OVERRIDE;
-  virtual void OnFailureOrCancel() OVERRIDE;
 
   // Saves the image data to a file.
   void SaveToFile();
 
   // Sets wallpaper to the decoded image.
   void SetDecodedWallpaper();
 
   // Layout of the downloaded wallpaper.
   ash::WallpaperLayout layout_;
 
@@ skipping 17 matching lines @@
   WallpaperSetCustomWallpaperFunction();
 
  protected:
   virtual ~WallpaperSetCustomWallpaperFunction();
 
   // AsyncExtensionFunction overrides.
   virtual bool RunImpl() OVERRIDE;
 
  private:
   virtual void OnWallpaperDecoded(const gfx::ImageSkia& wallpaper) OVERRIDE;
-  virtual void OnFailureOrCancel() OVERRIDE;
 
   // Layout of the downloaded wallpaper.
   ash::WallpaperLayout layout_;
 
   // Email address of logged in user.
   std::string email_;
 
   // String representation of downloaded wallpaper.
   std::string image_data_;
 };
 
 #endif  // CHROME_BROWSER_CHROMEOS_EXTENSIONS_WALLPAPER_PRIVATE_API_H_