Add support for accelerated fixed root background

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 141 matching lines @@
 }
 
 void ScrollingCoordinator::updateLayerPositionConstraint(RenderLayer* layer)
 {
     ASSERT(layer->backing());
     RenderLayerBacking* backing = layer->backing();
     GraphicsLayer* mainLayer = backing->childForSuperlayers();
 
     // Avoid unnecessary commits
     clearPositionConstraintExceptForLayer(backing->ancestorClippingLayer(), mainLayer);
-    clearPositionConstraintExceptForLayer(backing->contentsContainmentLayer(), mainLayer);
     clearPositionConstraintExceptForLayer(backing->graphicsLayer(), mainLayer);
 
     if (WebLayer* scrollableLayer = scrollingWebLayerForGraphicsLayer(mainLayer))
         scrollableLayer->setPositionConstraint(computePositionConstraint(layer));
 }
 
 void ScrollingCoordinator::willDestroyScrollableArea(ScrollableArea* scrollableArea)
 {
     removeWebScrollbarLayer(scrollableArea, HorizontalScrollbar);
     removeWebScrollbarLayer(scrollableArea, VerticalScrollbar);
@@ skipping 461 matching lines @@
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(mainThreadScrollingReasons());
 }
 
 } // namespace WebCore