Redraw only dirty area for accelerated 2D Canvas.

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index e8805396aa094994d3774af6d021dc1439dbcf1d..eb1ebcbc27c39a6ebfdcfaf9c646931da18accab 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -219,6 +219,7 @@ void HTMLCanvasElement::didDraw(const FloatRect& 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);