Add UMA for service worker navigation preload.

content/browser/service_worker/service_worker_fetch_dispatcher.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_FETCH_DISPATCHER_H_
 #define CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_FETCH_DISPATCHER_H_
 
 #include <memory>
 
 #include "base/callback.h"
@@ skipping 32 matching lines @@
       std::unique_ptr<ServiceWorkerFetchRequest> request,
       ServiceWorkerVersion* version,
       ResourceType resource_type,
       const base::Optional<base::TimeDelta>& timeout,
       const net::NetLogWithSource& net_log,
       const base::Closure& prepare_callback,
       const FetchCallback& fetch_callback);
   ~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
   // |prepare_callback| as an intermediate step once the version is activated
   // and running.
   void Run();
 
  private:
   class ResponseCallback;
 
@@ skipping 27 matching lines @@
   mojom::FetchEventPreloadHandlePtr preload_handle_;
 
   base::WeakPtrFactory<ServiceWorkerFetchDispatcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ServiceWorkerFetchDispatcher);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_FETCH_DISPATCHER_H_