gfx::ImageSkia: Added a note about using this class vs ImageFamily.

ui/gfx/image/image_skia.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_H_
 #define UI_GFX_IMAGE_IMAGE_SKIA_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 15 matching lines @@
 class TestOnThread;
 }
 
 // Container for the same image at different densities, similar to NSImage.
 // Image height and width are in DIP (Density Indepent Pixel) coordinates.
 //
 // ImageSkia should be used whenever possible instead of SkBitmap.
 // Functions that mutate the image should operate on the gfx::ImageSkiaRep
 // returned from ImageSkia::GetRepresentation, not on ImageSkia.
 //
+// NOTE: This class should *not* be used to store multiple logical sizes of an
+// image (e.g., small, medium and large versions of an icon); use an ImageFamily
+// for that. An ImageSkia represents an image of a single logical size, with
+// potentially many different densities for high-DPI displays.
+//
 // ImageSkia is cheap to copy and intentionally supports copy semantics.
 class GFX_EXPORT ImageSkia {
  public:
   typedef std::vector<ImageSkiaRep> ImageSkiaReps;
 
   // Creates an instance with no bitmaps.
   ImageSkia();
 
   // Creates an instance that will use the |source| to get the image
   // for scale factors. |size| specifes the size of the image in DIP.
@@ skipping 120 matching lines @@
   // so that other thread can access the storage.
   void DetachStorageFromThread();
 
   // A refptr so that ImageRepSkia can be copied cheaply.
   scoped_refptr<internal::ImageSkiaStorage> storage_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_SKIA_H_