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..c0d882d3901da91410eabf4b8c48ea3ffdae0018 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) {
+  if (!fired_events)
+    return;
+  int unhandled_ratio = (fired_events - handled_events) * 100 / fired_events;
+  UMA_HISTOGRAM_PERCENTAGE("ServiceWorker.UnhandledEventRatio",
+                           unhandled_ratio);
+}
+
 }  // namespace content