service worker: Add UMA for navigation preload response timing.

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 4d75b4d2b31881fb7d76d55c2e8f4fc10dfb95c3..934e4ee9eff6922bfc0f381240c2c2f478c9b2e4 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -317,6 +317,19 @@ class ServiceWorkerMetrics {
   // navigation preload request is to be sent.
   static void RecordNavigationPreloadRequestHeaderSize(size_t size);
 
+  // Records timings for the navigation preload response and how
+  // it compares to starting the worker.
+  // |worker_start| is the time it took to prepare an activated and running
+  // worker to receive the fetch event. |initial_worker_status| and
+  // |start_situation| describe the preparation needed.
+  // |response_start| is the time it took until the navigation preload response
+  // started.
+  CONTENT_EXPORT static void RecordNavigationPreloadResponse(
+      base::TimeDelta worker_start,
+      base::TimeDelta response_start,
+      EmbeddedWorkerStatus initial_worker_status,
+      StartSituation start_situation);
+
   // Records the result of trying to handle a request for a service worker
   // script.
   static void RecordContextRequestHandlerStatus(