Add UMA for how long service workers run for.

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 7603183776b2cb9a5dac6ce50c168e4c107f9d73..003427de85c47df4a25dd50d41f95a8bc66acda6 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -354,6 +354,8 @@ class ServiceWorkerMetrics {
       bool is_installed,
       bool is_main_script);
 
+  static void RecordRuntime(base::TimeDelta time);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ServiceWorkerMetrics);
 };