Add UMA to record the ratio of how many fetch events are not handled

content/browser/service_worker/service_worker_version.h

Index: content/browser/service_worker/service_worker_version.h
diff --git a/content/browser/service_worker/service_worker_version.h b/content/browser/service_worker/service_worker_version.h
index b020c25214b35b2b508811919be09e4d2cb0cde8..af011e563665c7204a35ff22f653eb996a6407c3 100644
--- a/content/browser/service_worker/service_worker_version.h
+++ b/content/browser/service_worker/service_worker_version.h
@@ -329,6 +329,8 @@ class CONTENT_EXPORT ServiceWorkerVersion
   FRIEND_TEST_ALL_PREFIXES(ServiceWorkerVersionBrowserTest,
                            TimeoutWorkerInEvent);
 
+  class ServiceWorkerEventMetrics;
+
   typedef ServiceWorkerVersion self;
   using ServiceWorkerClients = std::vector<ServiceWorkerClientInfo>;
 
@@ -553,6 +555,8 @@ class CONTENT_EXPORT ServiceWorkerVersion
   // running |start_callbacks_|.
   ServiceWorkerStatusCode start_worker_status_ = SERVICE_WORKER_OK;
 
+  scoped_ptr<ServiceWorkerEventMetrics> metrics_;
+
   base::WeakPtrFactory<ServiceWorkerVersion> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ServiceWorkerVersion);