Move overdraw tracking code from GraphicsContext to CanvasRenderingContext2D

Source/platform/graphics/GraphicsContextClient.h

-// Copyright 2015 The Chromium Authors. All rights reserved.
-//
-// The Chromium Authors can be found at
-// http://src.chromium.org/svn/trunk/src/AUTHORS
-//
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-// modification, are permitted provided that the following conditions are
-// met:
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-//    * Redistributions of source code must retain the above copyright
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-
-#ifndef GraphicsContextClient_h
-#define GraphicsContextClient_h
-
-#include "platform/PlatformExport.h"
-
-struct SkRect;
-class SkPaint;
-
-namespace blink {
-
-class GraphicsContext;
-
-class PLATFORM_EXPORT GraphicsContextClient {
-public:
-    enum DrawType {
-        UntransformedUnclippedFill,
-        Fill,
-        FillOrStroke
-    };
-
-    enum ImageType {
-        NoImage,
-        OpaqueImage,
-        NonOpaqueImage
-    };
-
-    void willDrawRect(GraphicsContext*, const SkRect&, const SkPaint*, ImageType, DrawType);
-    virtual ~GraphicsContextClient() { }
-protected:
-
-    // Called when the draw about to be executed will overwrite the entire canvas.
-    virtual void willOverwriteCanvas() = 0;
-};
-
-} // blink
-
-#endif