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

content/browser/service_worker/service_worker_metrics.cc

Index: content/browser/service_worker/service_worker_metrics.cc
diff --git a/content/browser/service_worker/service_worker_metrics.cc b/content/browser/service_worker/service_worker_metrics.cc
index dc3c157ab2cc9310699590c828902b3d442c97b4..c582cbcaf6d55a07138338f25250b6d118645c57 100644
--- a/content/browser/service_worker/service_worker_metrics.cc
+++ b/content/browser/service_worker/service_worker_metrics.cc
@@ -107,4 +107,13 @@ void ServiceWorkerMetrics::RecordInstallEventStatus(
                             SERVICE_WORKER_ERROR_MAX_VALUE);
 }
 
+void ServiceWorkerMetrics::RecordEventStatus(size_t fired_events,
+                                             size_t handled_events) {
+  int unhandled_ratio = 0;
+  if (handled_events > 0u)

[nhiroki, 2015/05/15 02:41:25]

This should be |fired_events| to avoid divining by 0.

[kinuko, 2015/05/15 04:42:23]

Done. I'm extremely careless these days.

+    unhandled_ratio = (fired_events - handled_events) * 100.0 / fired_events;
+  UMA_HISTOGRAM_PERCENTAGE("ServiceWorker.UnhandledEventRatio",
+                           unhandled_ratio);

[nhiroki, 2015/05/15 02:41:25]

Do we need to record in the case where |fired_events| is 0? This could record
noise because a version failed to start can go through here.

[kinuko, 2015/05/15 04:42:23]

Done.

+}
+
 }  // namespace content