Load 2x resources on demand

ui/gfx/image/image_skia_source.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 UI_GFX_IMAGE_IMAGE_SKIA_SOURCE_H_
 #define UI_GFX_IMAGE_IMAGE_SKIA_SOURCE_H_
 
 #include "ui/base/layout.h"
 #include "ui/base/ui_export.h"
 
 namespace gfx {
 
 class ImageSkiaRep;
 
 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;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_SKIA_SOURCE_H_