Set root clip layer not to maskToBounds.

Source/core/rendering/RenderLayerCompositor.h

 /*
  * Copyright (C) 2009 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 312 matching lines @@
     bool m_compositing;
     bool m_compositingLayersNeedRebuild;
     bool m_forceCompositingMode;
     bool m_inPostLayoutUpdate; // true when it's OK to trust layout information (e.g. layer sizes and positions)
     bool m_needsUpdateCompositingRequirementsState;
 
     bool m_isTrackingRepaints; // Used for testing.
     
     RootLayerAttachment m_rootLayerAttachment;
 
-    // Enclosing clipping layer for iframe content
-    OwnPtr<GraphicsLayer> m_clipLayer;
+    // Enclosing container layer, which clips for iframe content
+    OwnPtr<GraphicsLayer> m_containerLayer;
     OwnPtr<GraphicsLayer> m_scrollLayer;
 
     HashSet<RenderLayer*> m_viewportConstrainedLayers;
     HashSet<RenderLayer*> m_viewportConstrainedLayersNeedingUpdate;
 
     // This is used in updateCompositingRequirementsState to avoid full tree
     // walks while determining if layers have unclipped descendants.
     HashSet<RenderLayer*> m_outOfFlowPositionedLayers;
 
     // Enclosing layer for overflow controls and the clipping layer
@@ skipping 16 matching lines @@
     int m_secondaryCompositedLayerCount; // count of layers that have to be composited because of stacking or overlap.
     double m_obligatoryBackingStoreBytes;
     double m_secondaryBackingStoreBytes;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h