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

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

Index: third_party/WebKit/Source/core/events/Event.h
diff --git a/third_party/WebKit/Source/core/events/Event.h b/third_party/WebKit/Source/core/events/Event.h
index 3aadfc5cff162360dc751c41aab08c19d7bb9857..161b585e05e16248366e27b4e6af3d69b5f6398d 100644
--- a/third_party/WebKit/Source/core/events/Event.h
+++ b/third_party/WebKit/Source/core/events/Event.h
@@ -118,6 +118,12 @@ public:
     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; }