Add a setting to do frame scrolls through the root layer.

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 919 matching lines @@
         if (compositingState != PaintsIntoOwnBacking && compositingState != PaintsIntoGroupedBacking)
             return true;
     }
     return false;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     MainThreadScrollingReasons reasons = static_cast<MainThreadScrollingReasons>(0);
 
-    if (!m_page->settings().threadedScrollingEnabled())
+    // FIXME: make threaded scrolling work correctly with rootLayerScrolls.
+    if (!m_page->settings().threadedScrollingEnabled() || m_page->settings().rootLayerScrolls())
         reasons |= ThreadedScrollingDisabled;
 
     if (!m_page->mainFrame()->isLocalFrame())
         return reasons;
     FrameView* frameView = m_page->deprecatedLocalMainFrame()->view();
     if (!frameView)
         return reasons;
 
     if (frameView->hasSlowRepaintObjects())
         reasons |= HasSlowRepaintObjects;
@@ skipping 33 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : 0)
         return blink::WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink