Add UMA for service worker navigation preload.

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 319f0310f5e0aeee7d25a028e960450e2cf739ce..7f745ce3ce2de7a5eb24bebd63c41ab10ecc18eb 100644
--- a/content/browser/service_worker/service_worker_fetch_dispatcher.h
+++ b/content/browser/service_worker/service_worker_fetch_dispatcher.h
@@ -50,8 +50,8 @@ class CONTENT_EXPORT ServiceWorkerFetchDispatcher {
   ~ServiceWorkerFetchDispatcher();
 
   // If appropriate, starts the navigation preload request and creates
-  // |preload_handle_|.
-  void MaybeStartNavigationPreload(net::URLRequest* original_request);
+  // |preload_handle_|. Returns true if it started navigation preload.
+  bool MaybeStartNavigationPreload(net::URLRequest* original_request);
 
   // Dispatches a fetch event to the |version| given in ctor, and fires
   // |fetch_callback| (also given in ctor) when finishes. It runs