Add stable id to PaintImage.

third_party/WebKit/Source/platform/graphics/Image.h

 /*
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2004, 2005, 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 202 matching lines @@
 
  private:
   bool image_observer_disabled_;
   RefPtr<SharedBuffer> encoded_image_data_;
   // TODO(Oilpan): consider having Image on the Oilpan heap and
   // turn this into a Member<>.
   //
   // The observer (an ImageResourceContent) is an untraced member, with the
   // ImageResourceContent being responsible for clearing itself out.
   UntracedMember<ImageObserver> image_observer_;
+  PaintImage::Id stable_image_id_;
 };
 
 #define DEFINE_IMAGE_TYPE_CASTS(typeName)                          \
   DEFINE_TYPE_CASTS(typeName, Image, image, image->Is##typeName(), \
                     image.Is##typeName())
 
 }  // namespace blink
 
 #endif