Redraw only dirty area for accelerated 2D Canvas.

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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 201 matching lines @@
 
         m_dirtyRect.unite(r);
         ro->invalidatePaintRectangle(enclosingIntRect(m_dirtyRect));
     }
 
     notifyObserversCanvasChanged(rect);
 }
 
 void HTMLCanvasElement::notifyObserversCanvasChanged(const FloatRect& rect)
 {
+    m_dirtyRect = rect;

[Justin Novosad, 2014/06/20 14:06:53]

Storing the rect for later is a fragile approach because we are assuming that
CompositedLayerMapping::contentChanged will be called after this and before the
next call to this function.
Couldn't you propagate the dirtyRect in CanvasRenderingContext2D::didDraw
through the call to renderBox->contentChanged, which calls
RenderLayer->contentChanged, which calls CompositedLayerMapping->contentChange?

     WillBeHeapHashSet<RawPtrWillBeWeakMember<CanvasObserver> >::iterator end = m_observers.end();
     for (WillBeHeapHashSet<RawPtrWillBeWeakMember<CanvasObserver> >::iterator it = m_observers.begin(); it != end; ++it)
         (*it)->canvasChanged(this, rect);
 }
 
 void HTMLCanvasElement::reset()
 {
     if (m_ignoreReset)
         return;
 
@@ skipping 470 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }