Invalidate all wallpaper cache if display configuration changed

ash/desktop_background/user_wallpaper_delegate.h

Index: ash/desktop_background/user_wallpaper_delegate.h
diff --git a/ash/desktop_background/user_wallpaper_delegate.h b/ash/desktop_background/user_wallpaper_delegate.h
index e1e7f70e52b464e0e0309ad22cfed4380c2dbca0..7b37232759cb527004933b568c779f9a976efb99 100644
--- a/ash/desktop_background/user_wallpaper_delegate.h
+++ b/ash/desktop_background/user_wallpaper_delegate.h
@@ -31,8 +31,13 @@ class ASH_EXPORT UserWallpaperDelegate {
   virtual bool ShouldShowInitialAnimation() = 0;
 
   // Updates current wallpaper. It may switch the size of wallpaper based on the
-  // current display's resolution.
-  virtual void UpdateWallpaper() = 0;
+  // current display's resolution. If |clear_cache| is true, all wallpaper
+  // cache should be cleared. This is required when the display's native
+  // resolution changes to a larger resolution (e.g. when hooked up a large
+  // external display) and we need to load a larger resolution wallpaper for the
+  // large display. All the previous small resolution wallpaper cache should be
+  // cleared.
+  virtual void UpdateWallpaper(bool clear_cache) = 0;
 
   // Initialize wallpaper.
   virtual void InitializeWallpaper() = 0;