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

Source/core/testing/Internals.cpp

 /*
  * 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 1608 matching lines @@
     if (flags & LAYER_TREE_INCLUDES_TILE_CACHES)
         layerTreeFlags |= LayerTreeFlagsIncludeTileCaches;
     if (flags & LAYER_TREE_INCLUDES_REPAINT_RECTS)
         layerTreeFlags |= LayerTreeFlagsIncludeRepaintRects;
     if (flags & LAYER_TREE_INCLUDES_PAINTING_PHASES)
         layerTreeFlags |= LayerTreeFlagsIncludePaintingPhases;
 
     return document->frame()->layerTreeAsText(layerTreeFlags);
 }
 
+void Internals::setNeedsCompositedScrolling(Element* element, unsigned needsCompositedScrolling, ExceptionCode& ec)
+{
+    if (!element) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    element->document()->updateLayout();
+
+    RenderObject* renderer = element->renderer();
+    if (!renderer || !renderer->isBox()) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    RenderLayer* layer = toRenderBox(renderer)->layer();
+    if (!layer) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    layer->setForceNeedsCompositedScrolling(static_cast<RenderLayer::ForceNeedsCompositedScrollingMode>(needsCompositedScrolling));
+}
+
 String Internals::repaintRectsAsText(Document* document, ExceptionCode& ec) const
 {
     if (!document || !document->frame()) {
         ec = INVALID_ACCESS_ERR;
         return String();
     }
 
     return document->frame()->trackedRepaintRectsAsText();
 }
 
@@ skipping 358 matching lines @@
 
     RenderObject* renderer = select->renderer();
     if (!renderer->isMenuList())
         return false;
 
     RenderMenuList* menuList = toRenderMenuList(renderer);
     return menuList->popupIsVisible();
 }
 
 }