Redraw only dirty area for accelerated 2D Canvas.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index 0b01a059bc5024efec1bad6b25daa9ad93aab017..30a8bab287118186d4cebb9e80057b9fa617487d 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -1714,7 +1714,7 @@ void CompositedLayerMapping::contentChanged(ContentChangeType changeType)
     }
 
     if ((changeType == CanvasChanged || changeType == CanvasPixelsChanged) && isAcceleratedCanvas(renderer())) {
-        m_graphicsLayer->setContentsNeedsDisplay();
+        m_graphicsLayer->setContentsNeedsDisplayInRect(toHTMLCanvasElement(renderer()->node())->dirtyRect());
         return;

[abarth-chromium, 2014/06/20 15:19:47]

Rather than using this awkward notification pipeline, can you refactor the code
to call a function on CompositedLayerMapping that passes the dirtyRect as an
actual argument?  Currently, this sends the control flow through a one path and
the data through another path.  It would be much better to send the control and
data through the same path.  Also, that would save you from having to store
m_dirtyRect on HTMLCanvasElement.

     }
 }