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 89ab3d58553511a76855f07707c8145998c4b594..25e69d9394c0a0c2efd5b95e40ad00ec95ac93b7 100644
--- a/Source/core/testing/Internals.idl
+++ b/Source/core/testing/Internals.idl
@@ -177,6 +177,11 @@
     [RaisesException] NodeList paintOrderListBeforePromote(Element element);
     [RaisesException] NodeList paintOrderListAfterPromote(Element element);
 
+    const unsigned short DoNotForceCompositedScrolling = 0;
+    const unsigned short ForceCompositedScrollingOn = 1;
+    const unsigned short ForceCompositedScrollingOff = 2;

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

These values need to be in sync with RenderLayer so it would need a comment.

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

Done.

+    [RaisesException] void setNeedsCompositedScrolling(Element element, unsigned short value);
+
     [RaisesException] DOMString scrollingStateTreeAsText(Document document);
     [RaisesException] DOMString mainThreadScrollingReasons(Document document);
     [RaisesException] ClientRectList nonFastScrollableRects(Document document);