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

Source/core/testing/Internals.idl

Index: Source/core/testing/Internals.idl
diff --git a/Source/core/testing/Internals.idl b/Source/core/testing/Internals.idl
index 8db41cd41544b3e8b0147021b7fd7724f47afaf8..ea7552d07e862d2ae8cad4c811e9c4f2b1463748 100644
--- a/Source/core/testing/Internals.idl
+++ b/Source/core/testing/Internals.idl
@@ -169,6 +169,12 @@
     [RaisesException] NodeList paintOrderListBeforePromote(Element element);
     [RaisesException] NodeList paintOrderListAfterPromote(Element element);
 
+    // The values of these constants must be kept in sync with those in RenderLayer.
+    const unsigned short DoNotForceCompositedScrolling = 0;
+    const unsigned short CompositedScrollingAlwaysOn = 1;
+    const unsigned short CompositedScrollingAlwaysOff = 2;
+    [RaisesException] void setNeedsCompositedScrolling(Element element, unsigned short value);
+
     [RaisesException] DOMString scrollingStateTreeAsText(Document document);
     [RaisesException] DOMString mainThreadScrollingReasons(Document document);
     [RaisesException] ClientRectList nonFastScrollableRects(Document document);