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

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

 /*
  * 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, 2004, 2005, 2006, 2007, 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 100 matching lines @@
 
     const AtomicString& type() const { return m_type; }
     void setType(const AtomicString& type) { m_type = type; }
 
     EventTarget* target() const { return m_target.get(); }
     void setTarget(EventTarget*);
 
     EventTarget* currentTarget() const;
     void setCurrentTarget(EventTarget* currentTarget) { m_currentTarget = currentTarget; }
 
+    // This callback is invoked when an event listener has been dispatched
+    // at the current target. It should only be used to influence UMA metrics
+    // and not change functionality since observing the presence of listeners
+    // is dangerous.
+    virtual void doneDispatchingEventAtCurrentTarget() {}
+
     unsigned short eventPhase() const { return m_eventPhase; }
     void setEventPhase(unsigned short eventPhase) { m_eventPhase = eventPhase; }
 
     bool bubbles() const { return m_canBubble; }
     bool cancelable() const { return m_cancelable; }
     bool scoped() const { return m_scoped; }
     bool relatedTargetScoped() const { return m_relatedTargetScoped; }
 
     // Event creation timestamp in milliseconds. If |HiResEventTimeStamp|
     // runtime feature is enabled it returns a DOMHighResTimeStamp using the
@@ skipping 124 matching lines @@
     // WebInputEvent instance.
     double m_platformTimeStamp;
 };
 
 #define DEFINE_EVENT_TYPE_CASTS(typeName) \
     DEFINE_TYPE_CASTS(typeName, Event, event, event->is##typeName(), event.is##typeName())
 
 } // namespace blink
 
 #endif // Event_h