Add a setting to disable threaded scrolling

Source/core/page/scrolling/ScrollingCoordinator.h

 /*
  * 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 69 matching lines @@
 
 #if OS(MACOSX)
     // Dispatched by the scrolling tree during handleWheelEvent. This is required as long as scrollbars are painted on the main thread.
     void handleWheelEventPhase(PlatformWheelEventPhase);
 #endif
 
     enum MainThreadScrollingReasonFlags {
         HasSlowRepaintObjects = 1 << 0,
         HasViewportConstrainedObjectsWithoutSupportingFixedLayers = 1 << 1,
         HasNonLayerViewportConstrainedObjects = 1 << 2,
+        ThreadedScrollingDisabled = 1 << 3
     };
 
     MainThreadScrollingReasons mainThreadScrollingReasons() const;
     bool shouldUpdateScrollLayerPositionOnMainThread() const { return mainThreadScrollingReasons() != 0; }
 
     PassOwnPtr<blink::WebScrollbarLayer> createSolidColorScrollbarLayer(ScrollbarOrientation, int thumbThickness, int trackStart, bool isLeftSideVerticalScrollbar);
 
     void willDestroyScrollableArea(ScrollableArea*);
     // Returns true if the coordinator handled this change.
     bool scrollableAreaScrollLayerDidChange(ScrollableArea*);
@@ skipping 51 matching lines @@
     HashSet<const RenderLayer*> m_layersWithTouchRects;
     bool m_wasFrameScrollable;
 
     // This is retained for testing.
     MainThreadScrollingReasons m_lastMainThreadScrollingReasons;
 };
 
 } // namespace blink
 
 #endif // ScrollingCoordinator_h