Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 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 733 matching lines @@
         m_scrollingContentsLayer->setOffsetFromRenderer(scrollingContentsOffset, GraphicsLayer::DontSetNeedsDisplay);
 
         if (m_foregroundLayer) {
             if (m_foregroundLayer->size() != m_scrollingContentsLayer->size())
                 m_foregroundLayer->setSize(m_scrollingContentsLayer->size());
             m_foregroundLayer->setNeedsDisplay();
             m_foregroundLayer->setOffsetFromRenderer(m_scrollingContentsLayer->offsetFromRenderer());
         }
     }
 
+    m_owningLayer->positionOverflowControls();
+
     // If this layer was created just for clipping or to apply perspective, it doesn't need its own backing store.
     setRequiresOwnBackingStore(compositor()->requiresOwnBackingStore(m_owningLayer, compAncestor));
 
     updateContentsRect(isSimpleContainer);
     updateBackgroundColor(isSimpleContainer);
     updateDrawsContent(isSimpleContainer);
     updateContentsOpaque();
     updateAfterWidgetResize();
     registerScrollingLayers();
 
@@ skipping 1210 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore