Redraw only dirty area for accelerated 2D Canvas.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

Index: Source/core/html/canvas/CanvasRenderingContext2D.cpp
diff --git a/Source/core/html/canvas/CanvasRenderingContext2D.cpp b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
index ccca4ddd03fc0da373af9092d78ca687cfec0e49..0b66f7451d663b9dd7adbb5ece5220afd9dae348 100644
--- a/Source/core/html/canvas/CanvasRenderingContext2D.cpp
+++ b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
@@ -1744,7 +1744,7 @@ void CanvasRenderingContext2D::didDraw(const FloatRect& dirtyRect)
     if (isAccelerated()) {
         RenderBox* renderBox = canvas()->renderBox();

[jamesr, 2014/06/23 17:41:44]

you aren't using this any more, no need to grab it

         if (renderBox && renderBox->hasAcceleratedCompositing()) {

[abarth-chromium, 2014/06/23 17:09:04]

I wonder if we can replace this call to hasAcceleratedCompositing with just
checking the platformLayer() for null.

-            renderBox->contentChanged(CanvasPixelsChanged);
+            platformLayer()->invalidateRect(dirtyRect);
             canvas()->clearCopiedImage();
             canvas()->notifyObserversCanvasChanged(dirtyRect);
             return;