Implement CSSOM smooth scroll for Elements

Source/platform/scroll/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 180 matching lines @@
 
     // NOTE: Only called from Internals for testing.
     void setScrollOffsetFromInternals(const IntPoint&);
 
     IntPoint clampScrollPosition(const IntPoint&) const;
 
     // Let subclasses provide a way of asking for and servicing scroll
     // animations.
     bool scheduleAnimation();
     void serviceScrollAnimations(double monotonicTime);
+    virtual void registerForAnimation() { }
+    virtual void deregisterForAnimation() { }
 
     virtual bool usesCompositedScrolling() const { return false; }
 
     // Returns true if the GraphicsLayer tree needs to be rebuilt.
     virtual bool updateAfterCompositingChange() { return false; }
 
     virtual bool userInputScrollable(ScrollbarOrientation) const = 0;
     virtual bool shouldPlaceVerticalScrollbarOnLeft() const = 0;
 
     // Convenience functions
@@ skipping 98 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 blink
 
 #endif // ScrollableArea_h