Clean up the way layout tests force elements to opt in/out of composited scrolling.

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 2b351acbe876fdb40e1a990f3556dc44d95b4dc3..6b1d625869c7c26fc4a934cbeb41f804a46bed44 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -867,6 +867,13 @@ public:
     enum PaintOrderListType {BeforePromote, AfterPromote};
     void computePaintOrderList(PaintOrderListType type, Vector<RefPtr<Node> >&);
 
+    enum ForceNeedsCompositedScrollingMode {
+        DoNotForceCompositedScrolling = 0,
+        CompositedScrollingAlwaysOn = 1,
+        CompositedScrollingAlwaysOff = 2
+    };
+    void setForceNeedsCompositedScrolling(ForceNeedsCompositedScrollingMode);
+
 private:
     enum CollectLayersBehavior { StopAtStackingContexts, StopAtStackingContainers };
 
@@ -914,6 +921,7 @@ private:
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void updateNeedsCompositedScrolling();
+    void didUpdateNeedsCompositedScrolling();
 
     // Returns true if the position changed.
     bool updateLayerPosition();
@@ -1298,6 +1306,8 @@ protected:
 
     CompositingProperties m_compositingProperties;
 
+    ForceNeedsCompositedScrollingMode m_forceNeedsCompositedScrolling;
+
 private:
     IntRect m_blockSelectionGapsBounds;