Redraw only dirty area for accelerated 2D Canvas.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1689 matching lines @@
     return false;
 }
 
 void CompositedLayerMapping::contentChanged(ContentChangeType changeType)
 {
     if ((changeType == ImageChanged) && isDirectlyCompositedImage()) {
         updateImageContents();
         return;
     }
 
-    if ((changeType == CanvasChanged || changeType == CanvasPixelsChanged) && isAcceleratedCanvas(renderer())) {
+    if (changeType == CanvasChanged && isAcceleratedCanvas(renderer())) {
         m_graphicsLayer->setContentsNeedsDisplay();
         return;
     }
 }
 
 void CompositedLayerMapping::updateImageContents()
 {
     ASSERT(renderer()->isImage());
     RenderImage* imageRenderer = toRenderImage(renderer());
 
@@ skipping 510 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore