Only update composited-scrolling state once after layout.

Source/core/platform/ScrollableArea.h

 /*
  * Copyright (C) 2008, 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 153 matching lines @@
     void setScrollOffsetFromInternals(const IntPoint&);
 
     IntPoint clampScrollPosition(const IntPoint&) const;
 
     // Let subclasses provide a way of asking for and servicing scroll
     // animations.
     virtual bool scheduleAnimation() { return false; }
     void serviceScrollAnimations();
 
     virtual bool usesCompositedScrolling() const { return false; }
+    virtual void updateNeedsCompositedScrolling() { }
 
     virtual void reportMemoryUsage(MemoryObjectInfo*) const;
 
     // Convenience functions
     int scrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? scrollPosition().x() : scrollPosition().y(); }
     int minimumScrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? minimumScrollPosition().x() : minimumScrollPosition().y(); }
     int maximumScrollPosition(ScrollbarOrientation orientation) { return orientation == HorizontalScrollbar ? maximumScrollPosition().x() : maximumScrollPosition().y(); }
     int clampScrollPosition(ScrollbarOrientation orientation, int pos)  { return std::max(std::min(pos, maximumScrollPosition(orientation)), minimumScrollPosition(orientation)); }
 
 protected:
@@ skipping 51 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace WebCore
 
 #endif // ScrollableArea_h