Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/skia/OpaqueRegionSkia.h

 /*
  * Copyright (c) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 39 matching lines @@
     OpaqueRegionSkia();
 
     // The resulting opaque region as a single rect.
     IntRect asRect() const;
 
     void pushCanvasLayer(const SkPaint*);
     void popCanvasLayer(const GraphicsContext*);
 
     void setImageMask(const SkRect& imageOpaqueRect);
 
+    // Set this to true to track regions that occlude the destination instead of only regions that produce opaque pixels.
+    void setTreatOverwriteAsOpaque(bool value) { m_treatOverwriteAsOpaque = value; }
+
     enum DrawType {
         FillOnly,
         FillOrStroke
     };
 
     void didDrawRect(const GraphicsContext*, const SkRect&, const SkPaint&, const SkBitmap* sourceBitmap);
     void didDrawPath(const GraphicsContext*, const SkPath&, const SkPaint&);
     void didDrawPoints(const GraphicsContext*, SkCanvas::PointMode, int numPoints, const SkPoint[], const SkPaint&);
     void didDrawBounded(const GraphicsContext*, const SkRect&, const SkPaint&);
     void didDrawUnbounded(const GraphicsContext*, const SkPaint&, DrawType);
@@ skipping 19 matching lines @@
 private:
     void didDraw(const GraphicsContext*, const SkRect&, const SkPaint&, const SkBitmap* sourceBitmap, bool fillsBounds, DrawType);
     void applyOpaqueRegionFromLayer(const GraphicsContext*, const SkRect& layerOpaqueRect, const SkPaint&);
     void markRectAsOpaque(const SkRect&);
     void markRectAsNonOpaque(const SkRect&);
     void markAllAsNonOpaque();
 
     SkRect& currentTrackingOpaqueRect();
 
     SkRect m_opaqueRect;
+    bool m_treatOverwriteAsOpaque;
 
     Vector<CanvasLayerState, 3> m_canvasLayerStack;
 };
 
 }
 #endif // OpaqueRegionSkia_h