Add UMA metric for tracking root level listeners for blocking touch.

third_party/WebKit/Source/core/events/Event.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 236 matching lines @@
 void Event::setTarget(EventTarget* target)
 {
     if (m_target == target)
         return;
 
     m_target = target;
     if (m_target)
         receivedTarget();
 }
 
+void Event::doneDispatchingEventAtCurrentTarget()
+{
+}
+
 void Event::receivedTarget()
 {
 }
 
 void Event::setUnderlyingEvent(Event* ue)
 {
     // Prohibit creation of a cycle -- just do nothing in that case.
     for (Event* e = ue; e; e = e->underlyingEvent())
         if (e == this)
             return;
@@ skipping 102 matching lines @@
 
 DEFINE_TRACE(Event)
 {
     visitor->trace(m_currentTarget);
     visitor->trace(m_target);
     visitor->trace(m_underlyingEvent);
     visitor->trace(m_eventPath);
 }
 
 } // namespace blink