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 85a42d0e6ee230365f3bb8e06b46ce9038eae0af..19c6c15d05881727813f9916c031c256f618c425 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -713,6 +713,7 @@ public:
     RenderLayerBacking* backing() const { return m_backing.get(); }
     RenderLayerBacking* ensureBacking();
     void clearBacking(bool layerBeingDestroyed = false);
+    bool adjustForForceCompositedScrollingMode(bool) const;
     bool needsCompositedScrolling() const;
     bool needsToBeStackingContainer() const;
 
@@ -837,6 +838,7 @@ 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);
@@ -865,7 +867,8 @@ private:
 
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
-    void setNeedsCompositedScrolling(bool);
+    bool setNeedsCompositedScrolling(bool);
+    bool setNeedsToBeStackingContainer(bool);
     void didUpdateNeedsCompositedScrolling();
 
     // Returns true if the position changed.
@@ -1129,11 +1132,13 @@ protected:
     unsigned m_isUnclippedDescendant : 1;
 
     unsigned m_needsCompositedScrolling : 1;
-    unsigned m_needsCompositedScrollingHasBeenRecorded : 1;
+    unsigned m_needsToBeStackingContainerHasBeenRecorded : 1;
     unsigned m_willUseCompositedScrollingHasBeenRecorded : 1;
 
     unsigned m_isScrollableAreaHasBeenRecorded : 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
     // able to safely become a stacking context.