Make composited scrolling codepaths co-operate.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 6de6899d1411cdd671a2de0810127eb1c23f4c31..c22b2f1d880105e45b20bdf31c54d5bc204ffb09 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -846,6 +846,7 @@ private:
     bool acceleratedCompositingForOverflowScrollEnabled() const;
     // FIXME: This is a temporary flag and should be removed once accelerated
     // overflow scroll is ready (crbug.com/254111).
+    bool useCompositorDrivenAcceleratedScrolling() const;
     bool compositorDrivenAcceleratedScrollingEnabled() const;
     void updateCanBeStackingContainer();
     void collectBeforePromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListBeforePromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListBeforePromote);
@@ -875,6 +876,10 @@ private:
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void setNeedsCompositedScrolling(bool);
+    void setNeedsToBeStackingContainer(bool needsToBeStackingContainer)
+    {
+        m_needsToBeStackingContainer = needsToBeStackingContainer;
+    }
     void didUpdateNeedsCompositedScrolling();
 
     // Returns true if the position changed.
@@ -1150,6 +1155,7 @@ protected:
     unsigned m_isUnclippedDescendant : 1;
 
     unsigned m_needsCompositedScrolling : 1;
+    unsigned m_needsToBeStackingContainer : 1;
 
     // If this is true, then no non-descendant appears between any of our
     // descendants in stacking order. This is one of the requirements of being