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

content/browser/service_worker/service_worker_metrics.cc

 // 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.
 
 #include "content/browser/service_worker/service_worker_metrics.h"
 
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics_action.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/content_browser_client.h"
@@ skipping 89 matching lines @@
   UMA_HISTOGRAM_ENUMERATION("ServiceWorker.ActivateEventStatus", status,
                             SERVICE_WORKER_ERROR_MAX_VALUE);
 }
 
 void ServiceWorkerMetrics::RecordInstallEventStatus(
     ServiceWorkerStatusCode status) {
   UMA_HISTOGRAM_ENUMERATION("ServiceWorker.InstallEventStatus", status,
                             SERVICE_WORKER_ERROR_MAX_VALUE);
 }
 
+void ServiceWorkerMetrics::RecordEventStatus(unsigned int fired_events,
+                                             unsigned int handled_events) {
+  int unhandled_ratio = static_cast<double>(fired_events - handled_events) *
+                        100.0 / static_cast<double>(fired_events);

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

Nit: I don't think you actually need the double cast, you can just multiply by
100 (i..e. int) and divide by fired_events. The resulting value is logged as an
int anyway, and you're not doing any rounding anyway.

Though, you should probably guard against fired_events being zero, or you can
get a div by zero crash.

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

Done.
Aha, that's right...  done.

+  UMA_HISTOGRAM_PERCENTAGE("ServiceWorker.UnhandledEventRatio",
+                           unhandled_ratio);
+}
+
 }  // namespace content