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

Source/platform/graphics/GraphicsContext.h

 /*
  * Copyright (C) 2003, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008-2009 Torch Mobile, Inc.
  * Copyright (C) 2013 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 206 matching lines @@
     // possible quality.
     bool printing() const { return m_printing; }
     void setPrinting(bool printing) { m_printing = printing; }
 
     bool isAccelerated() const { return m_accelerated; }
     void setAccelerated(bool accelerated) { m_accelerated = accelerated; }
 
     // The opaque region is empty until tracking is turned on.
     // It is never clerared by the context.
     void setTrackOpaqueRegion(bool track) { m_trackOpaqueRegion = track; }
+    void setTreatOverwriteAsOpaque(bool value) { m_opaqueRegion.setTreatOverwriteAsOpaque(value); }

[Stephen White, 2014/07/25 19:29:06]

As discussed, let's make this a setter that specifies the mode of the region
tracker, since the difference is subtle but important. Something like:

setTrackRegion({OPAQUE | OVERWRITE | NONE});

     const OpaqueRegionSkia& opaqueRegion() const { return m_opaqueRegion; }
 
     // The text region is empty until tracking is turned on.
     // It is never clerared by the context.
     void setTrackTextRegion(bool track) { m_trackTextRegion = track; }
     const SkRect& textRegion() const { return m_textRegion; }
 
     bool updatingControlTints() const { return m_updatingControlTints; }
     void setUpdatingControlTints(bool updatingTints) { m_updatingControlTints = updatingTints; }
 
@@ skipping 300 matching lines @@
     bool m_updatingControlTints : 1;
     bool m_accelerated : 1;
     bool m_isCertainlyOpaque : 1;
     bool m_printing : 1;
     bool m_antialiasHairlineImages : 1;
 };
 
 } // namespace blink
 
 #endif // GraphicsContext_h