Add development Blink setting for impl-side rubber-banding

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 167 matching lines @@
 
     virtual bool shouldSuspendScrollAnimations() const { return true; }
     virtual void scrollbarStyleChanged() { }
 
     virtual bool scrollbarsCanBeActive() const = 0;
 
     // Returns the bounding box of this scrollable area, in the coordinate system of the enclosing scroll view.
     virtual IntRect scrollableAreaBoundingBox() const = 0;
 
     virtual bool isRubberBandInProgress() const { return false; }
+    virtual bool rubberBandingOnCompositorThread() const { return false; }
 
     virtual bool scrollAnimatorEnabled() const { return false; }
 
     // 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.
@@ skipping 110 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