Only update composited scrolling state when necessary

Source/core/rendering/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 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 2572 matching lines @@
 }
 
 static bool isRootmostFixedOrStickyLayer(RenderLayer* layer)
 {
     if (layer->renderer()->isStickyPositioned())
         return true;
 
     if (layer->renderer()->style()->position() != FixedPosition)
         return false;
 
-    for (RenderLayer* stackingContainer = layer->stackingContainer(); stackingContainer; stackingContainer = stackingContainer->stackingContainer()) {
+    for (RenderLayer* stackingContainer = layer->ancestorStackingContainer(); stackingContainer; stackingContainer = stackingContainer->ancestorStackingContainer()) {
         if (stackingContainer->isComposited() && stackingContainer->renderer()->style()->position() == FixedPosition)
             return false;
     }
 
     return true;
 }
 
 void RenderLayerCompositor::updateViewportConstraintStatus(RenderLayer* layer)
 {
     if (isRootmostFixedOrStickyLayer(layer))
@@ skipping 112 matching lines @@
     info.addMember(m_layerForScrollCorner, "layerForScrollCorner");
 #if ENABLE(RUBBER_BANDING)
     info.addMember(m_layerForOverhangAreas, "layerForOverhangAreas");
     info.addMember(m_contentShadowLayer, "contentShadowLayer");
     info.addMember(m_layerForTopOverhangArea, "layerForTopOverhangArea");
     info.addMember(m_layerForBottomOverhangArea, "layerForBottomOverhangArea");
 #endif
 }
 
 } // namespace WebCore