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

content/browser/service_worker/service_worker_metrics.h

Index: content/browser/service_worker/service_worker_metrics.h
diff --git a/content/browser/service_worker/service_worker_metrics.h b/content/browser/service_worker/service_worker_metrics.h
index 7b477af24f23930db2ea0c25159bcc0fbc462ae6..400cd7eb7d46e5be81d6d77d3dd77815cb63aab5 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -65,6 +65,10 @@ class ServiceWorkerMetrics {
   static void RecordActivateEventStatus(ServiceWorkerStatusCode status);
   static void RecordInstallEventStatus(ServiceWorkerStatusCode status);
 
+  // Records the ratio of unhandled events to the all events fired during
+  // the lifetime of ServiceWorker.
+  static void RecordEventStatus(size_t fired_events, size_t handled_events);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ServiceWorkerMetrics);
 };