service worker: Add UMA for navigation preload response timing.

content/browser/service_worker/service_worker_url_request_job.cc

Index: content/browser/service_worker/service_worker_url_request_job.cc
diff --git a/content/browser/service_worker/service_worker_url_request_job.cc b/content/browser/service_worker/service_worker_url_request_job.cc
index f69a0ac376a99a62458a61cead649368117f6733..d274cf353bf3b4c0a573073cd1fc1502b9682d32 100644
--- a/content/browser/service_worker/service_worker_url_request_job.cc
+++ b/content/browser/service_worker/service_worker_url_request_job.cc
@@ -555,9 +555,11 @@ void ServiceWorkerURLRequestJob::DidPrepareFetchEvent(
   }
   if (version->should_exclude_from_uma())
     return;
+  worker_start_situation_ = version->embedded_worker()->start_situation();
   ServiceWorkerMetrics::RecordActivatedWorkerPreparationForMainFrame(
       worker_ready_time_ - request()->creation_time(), initial_worker_status_,
-      version->embedded_worker()->start_situation(), did_navigation_preload_);
+      worker_start_situation_, did_navigation_preload_);
+  MaybeReportNavigationPreloadMetrics();
 }
 
 void ServiceWorkerURLRequestJob::DidDispatchFetchEvent(
@@ -885,9 +887,35 @@ void ServiceWorkerURLRequestJob::RequestBodyFileSizesResolved(bool success) {
       base::Bind(&ServiceWorkerURLRequestJob::DidDispatchFetchEvent,
                  weak_factory_.GetWeakPtr())));
   worker_start_time_ = base::TimeTicks::Now();
-  did_navigation_preload_ =
-      fetch_dispatcher_->MaybeStartNavigationPreload(request());
+  did_navigation_preload_ = fetch_dispatcher_->MaybeStartNavigationPreload(
+      request(),
+      base::Bind(&ServiceWorkerURLRequestJob::OnNavigationPreloadResponse,
+                 weak_factory_.GetWeakPtr()));
   fetch_dispatcher_->Run();
 }
 
+void ServiceWorkerURLRequestJob::OnNavigationPreloadResponse() {
+  DCHECK(navigation_preload_response_time_.is_null());
+  navigation_preload_response_time_ = base::TimeTicks::Now();
+  MaybeReportNavigationPreloadMetrics();
+}
+
+void ServiceWorkerURLRequestJob::MaybeReportNavigationPreloadMetrics() {
+  if (worker_start_time_.is_null() || worker_ready_time_.is_null() ||
+      navigation_preload_response_time_.is_null()) {
+    return;
+  }
+  // Don't expect this to happen, but don't ruin the UMA if it does.

[horo, 2017/01/24 01:48:33]

Do we actually need this check??
I think DCHECK() is enough.

[falken, 2017/01/24 04:41:18]

I wasn't sure if OnReceivedResposne() is gauranteed to be called only once. I
revised it to use OnceCallback so the callback wouldn't be called again if that
happens.

+  if (reported_navigation_preload_metrics_) {
+    NOTREACHED();
+    return;
+  }
+  reported_navigation_preload_metrics_ = true;
+
+  ServiceWorkerMetrics::RecordNavigationPreloadResponse(
+      worker_ready_time_ - worker_start_time_,
+      navigation_preload_response_time_ - worker_start_time_,
+      initial_worker_status_, worker_start_situation_);
+}
+
 }  // namespace content