Add metric to know how user use scrollbar

third_party/WebKit/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 346 matching lines @@
   virtual bool shouldPerformScrollAnchoring() const { return false; }
 
   // Need to promptly let go of owned animator objects.
   EAGERLY_FINALIZE();
   DECLARE_VIRTUAL_TRACE();
 
   virtual void clearScrollableArea();
 
   virtual ScrollAnchor* scrollAnchor() { return nullptr; }
 
+  virtual void didScrollWithScrollbar(ScrollbarPart, ScrollbarOrientation) {}
+
  protected:
   ScrollableArea();
 
   ScrollbarOrientation scrollbarOrientationFromDirection(
       ScrollDirectionPhysical) const;
   float scrollStep(ScrollGranularity, ScrollbarOrientation) const;
 
   void setScrollOrigin(const IntPoint&);
   void resetScrollOriginChanged() { m_scrollOriginChanged = false; }
 
@@ skipping 63 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