Fix page jumping back to initial position after an overflow-bounce scroll.

Source/platform/mac/ScrollElasticityController.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 39 matching lines @@
     virtual bool canScrollVertically() = 0;
     virtual bool shouldRubberBandInDirection(ScrollDirection) = 0;
 
     // Return the absolute scroll position, not relative to the scroll origin.
     virtual WebCore::IntPoint absoluteScrollPosition() = 0;
 
     virtual void immediateScrollBy(const FloatSize&) = 0;
     virtual void immediateScrollByWithoutContentEdgeConstraints(const FloatSize&) = 0;
     virtual void startSnapRubberbandTimer() = 0;
     virtual void stopSnapRubberbandTimer() = 0;
+
+    // If the current scroll position is within the overhang area, this function will cause
+    // the page to scroll to the nearest boundary point.
+    virtual void adjustScrollPositionToBoundsIfNecessary() = 0;
 };
 
 class ScrollElasticityController {
     WTF_MAKE_NONCOPYABLE(ScrollElasticityController);
 
 public:
     explicit ScrollElasticityController(ScrollElasticityControllerClient*);
 
     bool handleWheelEvent(const PlatformWheelEvent&);
     void snapRubberBandTimerFired();
@@ skipping 24 matching lines @@
     FloatSize m_origVelocity;
 
     bool m_snapRubberbandTimerIsActive;
 };
 
 } // namespace WebCore
 
 #endif // USE(RUBBER_BANDING)
 
 #endif // ScrollElasticityController_h