Move overdraw tracking code from GraphicsContext to CanvasRenderingContext2D

Source/core/html/HTMLCanvasElement.h

Index: Source/core/html/HTMLCanvasElement.h
diff --git a/Source/core/html/HTMLCanvasElement.h b/Source/core/html/HTMLCanvasElement.h
index c98b4b62c233b7d78fc62aa285c96ad6ea05954b..517d613827d4e256df81918aedbaa6c01dfa1738 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -155,6 +155,7 @@ public:
     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;
@@ -166,6 +167,9 @@ public:
 
     virtual void trace(Visitor*) override;
 
+    // Methods used for testing
+    void createImageBufferUsingSurface(PassOwnPtr<ImageBufferSurface>);
+
 protected:
     virtual void didMoveToNewDocument(Document& oldDocument) override;
 
@@ -182,7 +186,7 @@ private:
     PassOwnPtr<RecordingImageBufferFallbackSurfaceFactory> createSurfaceFactory(const IntSize& deviceSize, int* msaaSampleCount) const;
     PassOwnPtr<ImageBufferSurface> createImageBufferSurface(const IntSize& deviceSize, int* msaaSampleCount);
     void createImageBuffer();
-    void createImageBufferInternal();
+    void createImageBufferInternal(PassOwnPtr<ImageBufferSurface> externalSurface);
     bool shouldUseDisplayList(const IntSize& deviceSize);
 
     void setSurfaceSize(const IntSize&);