Use low resolution wallpaper for small screens.

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 <string>
 
 #include "ash/ash_export.h"
 #include "third_party/skia/include/core/SkColor.h"
 
 namespace ash {
 
 enum WallpaperLayout {
   CENTER,
   CENTER_CROPPED,
   STRETCH,
   TILE,
 };
 
+enum WallpaperResolution {
+  LARGE,
+  SMALL
+};
+
+// Encapsulates wallpaper infomation needed by desktop background view.
+struct ASH_EXPORT WallpaperViewInfo {
+  int id;
+  WallpaperLayout layout;
+};
+
 struct ASH_EXPORT WallpaperInfo {
-  int id;
+  WallpaperViewInfo large;
+  WallpaperViewInfo small;
   int thumb_id;
-  WallpaperLayout 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;
 };
 
 const SkColor kLoginWallpaperColor = 0xFEFEFE;
 
 ASH_EXPORT int GetDefaultWallpaperIndex();
 ASH_EXPORT int GetGuestWallpaperIndex();
 ASH_EXPORT int GetInvalidWallpaperIndex();
 ASH_EXPORT WallpaperLayout GetLayoutEnum(const std::string& layout);
 ASH_EXPORT int GetNextWallpaperIndex(int index);
 ASH_EXPORT int GetSolidColorIndex();
 ASH_EXPORT int GetWallpaperCount();
 ASH_EXPORT const WallpaperInfo& GetWallpaperInfo(int index);
+ASH_EXPORT const WallpaperViewInfo& GetWallpaperViewInfo(int index,
+    WallpaperResolution resolution);
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_RESOURCES_H_