Add metric to know how user use scrollbar

third_party/WebKit/Source/platform/Widget.h

 /*
  * Copyright (C) 2004, 2005, 2006 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) 2008 Collabora Ltd.  All rights reserved.
  * Copyright (C) 2013 Google 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.
@@ skipping 14 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef Widget_h
 #define Widget_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/heap/Handle.h"
+#include "platform/scroll/ScrollTypes.h"
 #include "public/platform/WebFocusType.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
 class CullRect;
 class Event;
 class GraphicsContext;
 
 // The Widget class serves as a base class for FrameView, Scrollbar, and
@@ skipping 74 matching lines @@
   // Virtual methods to convert points to/from child widgets
   virtual IntPoint convertChildToSelf(const Widget*, const IntPoint&) const;
   virtual IntPoint convertSelfToChild(const Widget*, const IntPoint&) const;
 
   // Notifies this widget that it will no longer be receiving events.
   virtual void eventListenersRemoved() {}
 
   DECLARE_VIRTUAL_TRACE();
   virtual void dispose() {}
 
+  virtual void sendScrollbarUseUMA(ScrollbarPart, ScrollbarOrientation) {}

[bokan, 2017/01/17 21:24:01]

Rather than going through widget, I'd add a
ScrollableArea::didScrollWithScrollbar and implement it in
PaintInvalidationCapableScrollableArea which covers both
PaintLayerScrollableArea and FrameView.

+
  private:
   Member<Widget> m_parent;
   IntRect m_frameRect;
   bool m_selfVisible;
   bool m_parentVisible;
 };
 
 }  // namespace blink
 
 #endif  // Widget_h