Overlay scrollbars must respect ancestor clip

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index ea83bc9e465458e5b59d3e73b430a8b72a3eb3e3..e1e16c4813f1bc121efb0751b774007fd2100d48 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1537,6 +1537,14 @@ 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));