Enable user change background image in settings page in Aura build.

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_
+
+class SkBitmap;
+
+namespace ash {
+
+// This probably needs to put to another place. But I am not sure where is the
+// best place. It's basically util function which used both by ash and by
+// webui handler.
+extern const int kDefaultWallpaperResources[];
+extern const int kDefaultWallpaperResourcesThumb[];
+extern const int kDefaultWallpaperCount;
+
+const SkBitmap& GetDefaultWallpaper(int index);
+const SkBitmap& GetDefaultWallpaperThumbnail(int index);
+
+}  // namespace ash
+
+#endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_RESOURCES_H_