Redraw only dirty area for accelerated 2D Canvas.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 1724 matching lines @@
     return true;
 }
 
 void CanvasRenderingContext2D::didDraw(const FloatRect& dirtyRect)
 {
     if (dirtyRect.isEmpty())
         return;
 
     // If we are drawing to hardware and we have a composited layer, just call contentChanged().
     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;
         }
     }
 
     canvas()->didDraw(dirtyRect);
 }
 
 GraphicsContext* CanvasRenderingContext2D::drawingContext() const
@@ skipping 593 matching lines @@
     c->setAlphaAsFloat(1.0);
     c->clearShadow();
     c->setCompositeOperation(CompositeSourceOver, blink::WebBlendModeNormal);
     c->drawFocusRing(path, focusRingWidth, focusRingOutline, focusRingColor);
     c->restore();
     validateStateStack();
     didDraw(dirtyRect);
 }
 
 } // namespace WebCore