Fix smooth scroll overshooting when mouse held down in scrollbar track.

third_party/WebKit/Source/platform/scroll/Scrollbar.h

 /*
  * Copyright (C) 2004, 2006 Apple Computer, 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 204 matching lines @@
 
     float m_elasticOverscroll;
 
 private:
     bool isScrollbar() const override { return true; }
 
     void invalidate() override { setNeedsPaintInvalidation(AllParts); }
     void invalidateRect(const IntRect&) override { setNeedsPaintInvalidation(AllParts); }
 
     float scrollableAreaCurrentPos() const;
+    float scrollableAreaTargetPos() const;
+    bool thumbWillBeUnderMouse() const;
 
     bool m_trackNeedsRepaint;
     bool m_thumbNeedsRepaint;
 };
 
 DEFINE_TYPE_CASTS(Scrollbar, Widget, widget, widget->isScrollbar(), widget.isScrollbar());
 
 } // namespace blink
 
 #endif // Scrollbar_h