Replace the index mapping of wallpaper picker UI and hard coded wallpaper index in C++

ash/desktop_background/desktop_background_resources.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 ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_RESOURCES_H_
 #define ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_RESOURCES_H_
 
 #include "ash/ash_export.h"
 
 class SkBitmap;
@@ skipping 10 matching lines @@
 struct ASH_EXPORT WallpaperInfo {
   int id;
   int thumb_id;
   ImageLayout layout;
   // TODO(bshe): author member should be encoded to UTF16. We need to use i18n
   // string for this member after M19.
   const char* author;
   const char* website;
 };
 
+ASH_EXPORT int GetInvalidWallpaperIndex();
 ASH_EXPORT int GetDefaultWallpaperIndex();
 ASH_EXPORT int GetGuestWallpaperIndex();
 ASH_EXPORT int GetWallpaperCount();
 ASH_EXPORT const SkBitmap& GetWallpaper(int index);
-ASH_EXPORT const SkBitmap& GetWallpaperThumbnail(int index);
 ASH_EXPORT const WallpaperInfo& GetWallpaperInfo(int index);
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_RESOURCES_H_