Set up the infrastructure for Extension event metrics.

chrome/browser/extensions/api/mdns/mdns_api.cc

Index: chrome/browser/extensions/api/mdns/mdns_api.cc
diff --git a/chrome/browser/extensions/api/mdns/mdns_api.cc b/chrome/browser/extensions/api/mdns/mdns_api.cc
index 1dfa4ffe61c14ba94aa404724415c5514762eca4..8a34b75526ec83da9ed2ebda7f59426062be4eef 100644
--- a/chrome/browser/extensions/api/mdns/mdns_api.cc
+++ b/chrome/browser/extensions/api/mdns/mdns_api.cc
@@ -145,8 +145,8 @@ void MDnsAPI::OnDnsSdEvent(const std::string& service_type,
   }
 
   scoped_ptr<base::ListValue> results = mdns::OnServiceList::Create(args);
-  scoped_ptr<Event> event(
-      new Event(mdns::OnServiceList::kEventName, results.Pass()));
+  scoped_ptr<Event> event(new Event(
+      events::UNKNOWN, mdns::OnServiceList::kEventName, results.Pass()));
   event->restrict_to_browser_context = browser_context_;
   event->filter_info.SetServiceType(service_type);