Added UMA stats for extensions events on addListener.

chrome/browser/extensions/event_router.h

Index: chrome/browser/extensions/event_router.h
diff --git a/chrome/browser/extensions/event_router.h b/chrome/browser/extensions/event_router.h
index ec432e72b38f935aff0e08c61189a5f00cd3b80d..9973fbcafa200e429302379374e8e2eeb7d3e106 100644
--- a/chrome/browser/extensions/event_router.h
+++ b/chrome/browser/extensions/event_router.h
@@ -17,6 +17,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/values.h"
 #include "chrome/browser/extensions/event_listener_map.h"
+#include "chrome/browser/extensions/extension_event_histogram_value.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "extensions/common/event_filtering_info.h"
@@ -223,6 +224,8 @@ class EventRouter : public content::NotificationObserver,
   void DispatchPendingEvent(const linked_ptr<Event>& event,
                             ExtensionHost* host);
 
+  void RecordEventListenerAdded(const std::string& event_name);
+
   // Implementation of EventListenerMap::Delegate.
   virtual void OnListenerAdded(const EventListener* listener) OVERRIDE;
   virtual void OnListenerRemoved(const EventListener* listener) OVERRIDE;
@@ -233,6 +236,8 @@ class EventRouter : public content::NotificationObserver,
 
   EventListenerMap listeners_;
 
+  std::map<std::string, extensions::events::HistogramValue> histogram_values_;
+
   typedef base::hash_map<std::string, Observer*> ObserverMap;
   ObserverMap observers_;