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

content/browser/service_worker/service_worker_metrics.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_METRICS_H_
 #define CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_METRICS_H_
 
 #include "base/macros.h"
 #include "content/browser/service_worker/service_worker_database.h"
 
@@ skipping 47 matching lines @@
                                       bool is_installed);
 
   // Records the time taken to successfully start a worker. |is_installed|
   // indicates whether the version has been installed.
   static void RecordStartWorkerTime(const base::TimeDelta& time,
                                     bool is_installed);
 
   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(unsigned int fired_events,

[Alexei Svitkine (slow), 2015/05/14 17:49:59]

Nit: Chromium style doesn't usually use unsigned ints except when interfacing
with external APIs that require it. int is probably fine for this.

[kinuko, 2015/05/15 00:05:37]

Chromium guide says following sentence, is that what you mean?

'Do not use unsigned types to mean "this value should never be < 0".'

This is for counters, I think size_t is probably better? (Though int should be
also fine too)

+                                unsigned int handled_events);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ServiceWorkerMetrics);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_METRICS_H_