Redraw only dirty area for accelerated 2D Canvas.

Source/platform/graphics/GraphicsLayer.cpp

Index: Source/platform/graphics/GraphicsLayer.cpp
diff --git a/Source/platform/graphics/GraphicsLayer.cpp b/Source/platform/graphics/GraphicsLayer.cpp
index 6129a2e721fe347b557996cbb58f8afa24fb1ea7..57425c75fea223df716d22500f63841476e54c20 100644
--- a/Source/platform/graphics/GraphicsLayer.cpp
+++ b/Source/platform/graphics/GraphicsLayer.cpp
@@ -966,9 +966,14 @@ void GraphicsLayer::setIsRootForIsolatedGroup(bool isolated)
 
 void GraphicsLayer::setContentsNeedsDisplay()
 {
+    setContentsNeedsDisplayInRect(m_contentsRect);
+}
+
+void GraphicsLayer::setContentsNeedsDisplayInRect(const FloatRect& rect)
+{
     if (WebLayer* contentsLayer = contentsLayerIfRegistered()) {
-        contentsLayer->invalidate();
-        addRepaintRect(m_contentsRect);
+        contentsLayer->invalidateRect(rect);
+        addRepaintRect(rect);
     }
 }