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..8c34015d4931155f6df16c7f3db6851f73ab960d 100644
--- a/content/browser/service_worker/service_worker_metrics.cc
+++ b/content/browser/service_worker/service_worker_metrics.cc
@@ -107,4 +107,12 @@ void ServiceWorkerMetrics::RecordInstallEventStatus(
                             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