Move overdraw tracking code from GraphicsContext to CanvasRenderingContext2D

Source/core/html/HTMLImageElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 705 matching lines @@
     return HTMLImageFallbackHelper::customStyleForAltText(*this, style);
 }
 
 void HTMLImageElement::setUseFallbackContent()
 {
     m_useFallbackContent = true;
     if (document().inStyleRecalc())
         return;
     ensureUserAgentShadowRoot();
 }
+
+bool HTMLImageElement::isOpaque() const
+{
+    Image* image = const_cast<HTMLImageElement*>(this)->imageContents();
+    return image && image->currentFrameKnownToBeOpaque();
 }
+
+}