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..468c0ac0fb76c61abb0bf39b78e99f6aa664bb7e 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -126,6 +126,7 @@ public:
 
     void ensureUnacceleratedImageBuffer();
     ImageBuffer* buffer() const;
+    void createImageBufferUsingSurface(PassOwnPtr<ImageBufferSurface>); // For unit testing

[Stephen Chennney, 2015/02/11 14:33:20]

Nit: Could you put all test methods at the bottom of the public section.

[Justin Novosad, 2015/02/11 18:59:54]

Done.

     PassRefPtr<Image> copiedImage(SourceDrawingBuffer) const;
     void clearCopiedImage();
 
@@ -155,6 +156,7 @@ 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;
@@ -182,7 +184,7 @@ 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&);