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..d77fd2bd38515fb4a38686ce115e3b433a3c7266 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -65,6 +65,11 @@ 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(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);
 };