Overlay scrollbars must respect ancestor clip

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1519 matching lines @@
     LayoutPoint delta;
     convertToLayerCoords(ancestorLayer, delta);
     rect.move(-delta.x(), -delta.y());
 }
 
 void RenderLayer::didUpdateNeedsCompositedScrolling()
 {
     updateSelfPaintingLayer();
 }
 
+bool RenderLayer::needsToReparentOverflowControls() const
+{
+    return hasCompositedLayerMapping()
+        && scrollableArea()
+        && scrollableArea()->hasOverlayScrollbars()
+        && scrollableArea()->topmostScrollChild();
+}
+
 void RenderLayer::updateReflectionInfo(const RenderStyle* oldStyle)
 {
     ASSERT(!oldStyle || !renderer()->style()->reflectionDataEquivalent(oldStyle));
     if (renderer()->hasReflection()) {
         if (!m_reflectionInfo)
             m_reflectionInfo = adoptPtr(new RenderLayerReflectionInfo(*renderBox()));
         m_reflectionInfo->updateAfterStyleChange(oldStyle);
     } else if (m_reflectionInfo) {
         m_reflectionInfo = nullptr;
     }
@@ skipping 2240 matching lines @@
     }
 }
 
 void showLayerTree(const blink::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif