Load 2x resources on demand

ui/gfx/image/image_skia_source.h

Index: ui/gfx/image/image_skia_source.h
diff --git a/ui/gfx/image/image_skia_source.h b/ui/gfx/image/image_skia_source.h
index ed5e7ca8100660a2584a66e1f46d3be09daf8375..1fff1358717333866d16354ec68aef751bbf8b6e 100644
--- a/ui/gfx/image/image_skia_source.h
+++ b/ui/gfx/image/image_skia_source.h
@@ -16,7 +16,10 @@ class UI_EXPORT ImageSkiaSource {
  public:
   virtual ~ImageSkiaSource() {}
 
-  // Returns the ImageSkiaRep for the given |scale_factor|.
+  // Returns the ImageSkiaRep for the given |scale_factor|. ImageSkia
+  // caches the returned ImageSkiaRep and calls this method only if it
+  // doesn't have ImageSkaiRep for given |scale_factor|. There is
+  // no need for the implementation to cache the image.
   virtual gfx::ImageSkiaRep GetImageForScale(ui::ScaleFactor scale_factor) = 0;
 };