Move overdraw tracking code from GraphicsContext to CanvasRenderingContext2D

Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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.
@@ skipping 137 matching lines @@
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) override;
 
     // DocumentVisibilityObserver implementation
     virtual void didChangeVisibilityState(PageVisibilityState) override;
 
     // CanvasImageSource implementation
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const override;
     virtual bool wouldTaintOrigin(SecurityOrigin*) const override;
     virtual FloatSize sourceSize() const override;
     virtual bool isCanvasElement() const override { return true; }
+    virtual bool isOpaque() const override;
 
     // ImageBufferClient implementation
     virtual void notifySurfaceInvalid() override;
     virtual bool isDirty() override { return !m_dirtyRect.isEmpty(); }
     virtual void didFinalizeFrame() override;
     virtual void restoreCanvasMatrixClipStack() override;
 
     void doDeferredPaintInvalidation();
 
     virtual void trace(Visitor*) override;
@@ skipping 45 matching lines @@
     bool m_imageBufferIsClear;
     OwnPtr<ImageBuffer> m_imageBuffer;
     mutable OwnPtr<GraphicsContextStateSaver> m_contextStateSaver;
 
     mutable RefPtr<Image> m_copiedImage; // FIXME: This is temporary for platforms that have to copy the image buffer to render (and for CSSCanvasValue).
 };
 
 } // namespace blink
 
 #endif // HTMLCanvasElement_h