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

Source/core/testing/Internals.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 151 matching lines @@
     // Flags for layerTreeAsText.
     const unsigned short LAYER_TREE_INCLUDES_VISIBLE_RECTS = 1;
     const unsigned short LAYER_TREE_INCLUDES_TILE_CACHES = 2;
     const unsigned short LAYER_TREE_INCLUDES_REPAINT_RECTS = 4;
     const unsigned short LAYER_TREE_INCLUDES_PAINTING_PHASES = 8;
     [RaisesException] DOMString layerTreeAsText(Document document, optional unsigned short flags);
 
     [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);
 
     [RaisesException] DOMString repaintRectsAsText(Document document);
 
     [RaisesException] void garbageCollectDocumentResources(Document document);
 
     void allowRoundingHacks();
 
@@ skipping 55 matching lines @@
     void forceReload(boolean endToEnd);
 
     [Conditional=ENCRYPTED_MEDIA_V2] void initializeMockCDM();
 
     void enableMockSpeechSynthesizer();
 
     [RaisesException] DOMString getImageSourceURL(Element element);
 
     boolean isSelectPopupVisible(Node node);
 };