Overlay scrollbars must respect ancestor clip

Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 108 matching lines @@
                 overflowControlLayer->removeFromParent();
                 currentCompositedLayerMapping->parentForSublayers()->addChild(overflowControlLayer);
             }
         }
 
         if (shouldAppendLayer(layer))
             info.childLayersOfEnclosingCompositedLayer->append(currentCompositedLayerMapping->childForSuperlayers());
     }
 
     if (layer.scrollParent()
-        && layer.scrollParent()->hasCompositedLayerMapping()
-        && layer.scrollParent()->scrollableArea()->hasOverlayScrollbars()
+        && layer.scrollParent()->needsToReparentOverflowControls()
         && layer.scrollParent()->scrollableArea()->topmostScrollChild() == &layer)
         info.childLayersOfEnclosingCompositedLayer->append(layer.scrollParent()->compositedLayerMapping()->detachLayerForOverflowControls(*info.enclosingCompositedLayer));
 }
 
 }