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 18e40590e594cca3686cdc7101d31c4686587d3d..c2b3cd216b7a1d3a34c53f18a72b8316b51e1644 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -814,6 +814,13 @@ public:
     enum PaintOrderListType {BeforePromote, AfterPromote};
     void computePaintOrderList(PaintOrderListType type, Vector<RefPtr<Node> >&);
 
+    enum ForceNeedsCompositedScrollingMode {
+        DoNotForceCompositedScrolling = 0,
+        ForceCompositedScrollingOn,
+        ForceCompositedScrollingOff
+    };
+    void setForceNeedsCompositedScrolling(ForceNeedsCompositedScrollingMode mode);
+
 private:
     enum CollectLayersBehavior { StopAtStackingContexts, StopAtStackingContainers };
 
@@ -861,6 +868,7 @@ private:
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void updateNeedsCompositedScrolling();
+    void didUpdateNeedsCompositedScrolling();
 
     // Returns true if the position changed.
     bool updateLayerPosition();
@@ -1130,6 +1138,7 @@ protected:
     bool m_hasOutOfFlowPositionedDescendant : 1;
     bool m_hasOutOfFlowPositionedDescendantDirty : 1;
 
+    ForceNeedsCompositedScrollingMode m_forceNeedsCompositedScrolling;

[Julien - ping for review, 2013/05/06 22:56:59]

By mixing 2 different bitfield types here, you *will* make RenderLayer bigger on
Windows by preventing any bit packing.

We don't track memory consumption on RenderLayer as we do for other objects but
it would still be nice if you moved that after the bitfields (ideally, we should
move everything to unsigned to ensure proper padding)

[Ian Vollick, 2013/05/08 13:10:58]

Thank you for catching this! Noted and fixed.

     bool m_needsCompositedScrolling : 1;
 
     // If this is true, then no non-descendant appears between any of our