service worker: Add UMA for navigation preload response timing.

content/browser/service_worker/service_worker_fetch_dispatcher.h

Index: content/browser/service_worker/service_worker_fetch_dispatcher.h
diff --git a/content/browser/service_worker/service_worker_fetch_dispatcher.h b/content/browser/service_worker/service_worker_fetch_dispatcher.h
index 3064d7d3393939cb2e51c1ec44e5eb608bf223ae..06c4be91f9eb3ac60750a9698e201e58d6b232eb 100644
--- a/content/browser/service_worker/service_worker_fetch_dispatcher.h
+++ b/content/browser/service_worker/service_worker_fetch_dispatcher.h
@@ -52,7 +52,9 @@ class CONTENT_EXPORT ServiceWorkerFetchDispatcher {
 
   // If appropriate, starts the navigation preload request and creates
   // |preload_handle_|. Returns true if it started navigation preload.
-  bool MaybeStartNavigationPreload(net::URLRequest* original_request);
+  // |on_response| is invoked in OnReceiveResponse().
+  bool MaybeStartNavigationPreload(net::URLRequest* original_request,
+                                   base::Closure on_response);
 
   // Dispatches a fetch event to the |version| given in ctor, and fires
   // |fetch_callback| (also given in ctor) when finishes. It runs