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

ash/desktop_background/desktop_background_resources.h

Index: ash/desktop_background/desktop_background_resources.h
diff --git a/ash/desktop_background/desktop_background_resources.h b/ash/desktop_background/desktop_background_resources.h
index 0fde38e06afb01b89d62e2bd9cf5c357f7680f7e..3812f51d6826568a3aaf8758264fccc2fa2cada6 100644
--- a/ash/desktop_background/desktop_background_resources.h
+++ b/ash/desktop_background/desktop_background_resources.h
@@ -28,11 +28,11 @@ struct ASH_EXPORT WallpaperInfo {
   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