Add metric to know how user use scrollbar

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

 /*
  * Copyright (C) 2004, 2006, 2008 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 299 matching lines @@
   m_hoveredPart = part;
 }
 
 void Scrollbar::setPressedPart(ScrollbarPart part) {
   if (m_pressedPart != NoPart
       // When we no longer have a pressed part, we can start drawing a hovered
       // state on the hovered part.
       || m_hoveredPart != NoPart)
     setNeedsPaintInvalidation(
         static_cast<ScrollbarPart>(m_pressedPart | m_hoveredPart | part));
+
+  if (getScrollableArea())
+    getScrollableArea()->didScrollWithScrollbar(part, orientation());
+
   m_pressedPart = part;
 }
 
 bool Scrollbar::gestureEvent(const WebGestureEvent& evt,
                              bool* shouldUpdateCapture) {
   DCHECK(shouldUpdateCapture);
   switch (evt.type()) {
     case WebInputEvent::GestureTapDown: {
       IntPoint position = flooredIntPoint(evt.positionInRootFrame());
       setPressedPart(theme().hitTest(*this, position));
@@ skipping 299 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink