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

Source/core/testing/Internals.h

Index: Source/core/testing/Internals.h
diff --git a/Source/core/testing/Internals.h b/Source/core/testing/Internals.h
index 8eab70b8981e380db86c8c10d7e5aa239d52211d..8de214853997969770726abd83e330b16e092df8 100644
--- a/Source/core/testing/Internals.h
+++ b/Source/core/testing/Internals.h
@@ -214,8 +214,16 @@ public:
     String layerTreeAsText(Document*, unsigned flags, ExceptionCode&) const;
     String layerTreeAsText(Document*, ExceptionCode&) const;
 
-    PassRefPtr<NodeList> paintOrderListBeforePromote(Element* element, ExceptionCode& ec);
-    PassRefPtr<NodeList> paintOrderListAfterPromote(Element* element, ExceptionCode& ec);
+    PassRefPtr<NodeList> paintOrderListBeforePromote(Element*, ExceptionCode&);
+    PassRefPtr<NodeList> paintOrderListAfterPromote(Element*, ExceptionCode&);
+
+    enum {
+        // Values need to be kept in sync with Internals.idl.
+        DoNotForceCompositedScrolling = 0,
+        CompositedScrollingAlwaysOn = 1,
+        CompositedScrollingAlwaysOff = 2
+    };
+    void setNeedsCompositedScrolling(Element*, unsigned value, ExceptionCode&);
 
     String repaintRectsAsText(Document*, ExceptionCode&) const;
     String scrollingStateTreeAsText(Document*, ExceptionCode&) const;