Fix use-after-free in gfx::Image.

ui/gfx/image/image.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.
 
 // An Image wraps an image any flavor, be it platform-native GdkBitmap/NSImage,
 // or a SkBitmap. This also provides easy conversion to other image types
 // through operator overloading. It will cache the converted representations
 // internally to prevent double-conversion.
 //
 // The lifetime of both the initial representation and any converted ones are
@@ skipping 165 matching lines @@
 
  private:
   // Returns the type of the default representation.
   RepresentationType DefaultRepresentationType() const;
 
   // Returns the ImageRep of the appropriate type or NULL if there is no
   // representation of that type (and must_exist is false).
   internal::ImageRep* GetRepresentation(
       RepresentationType rep_type, bool must_exist) const;
 
-  // Stores a representation into the map.
+  // Stores a representation into the map. A representation of that type must
+  // not already be in the map.
   void AddRepresentation(scoped_ptr<internal::ImageRep> rep) const;
 
   // Internal class that holds all the representations. This allows the Image to
   // be cheaply copied.
   scoped_refptr<internal::ImageStorage> storage_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_H_