Reland "Use low resolution wallpaper for small screens"

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 54cdf55eed7fcf991c3af575bfcb8c67352e4609..1f776a5ef5f3f2289cc7f83b8e554881180b0119 100644
--- a/ash/desktop_background/desktop_background_resources.h
+++ b/ash/desktop_background/desktop_background_resources.h
@@ -19,10 +19,21 @@ enum WallpaperLayout {
   TILE,
 };
 
-struct ASH_EXPORT WallpaperInfo {
+enum WallpaperResolution {
+  LARGE,
+  SMALL
+};
+
+// Encapsulates wallpaper infomation needed by desktop background view.
+struct ASH_EXPORT WallpaperViewInfo {
   int id;
-  int thumb_id;
   WallpaperLayout layout;
+};
+
+struct ASH_EXPORT WallpaperInfo {
+  WallpaperViewInfo large_wallpaper;
+  WallpaperViewInfo small_wallpaper;
+  int thumb_id;
   // TODO(bshe): author member should be encoded to UTF16. We need to use i18n
   // string for this member after M19.
   const char* author;
@@ -39,6 +50,8 @@ 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