service worker: Add NavigationPreload runtime flag and skeleton interface

third_party/WebKit/Source/modules/serviceworkers/ServiceWorkerRegistration.idl

 // 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.
 
 // https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-obj
 [
     ActiveScriptWrappable,
     DependentLifetime,
     Exposed=(Window,Worker),
 ] interface ServiceWorkerRegistration : EventTarget {
     readonly attribute ServiceWorker? installing;
     readonly attribute ServiceWorker? waiting;
     readonly attribute ServiceWorker? active;
+    [RuntimeEnabled=ServiceWorkerNavigationPreload] readonly attribute NavigationPreloadManager navigationPreload;
 
     readonly attribute USVString scope;
 
     [CallWith=ScriptState] Promise<void> update();
     [CallWith=ScriptState] Promise<boolean> unregister();
 
     attribute EventHandler onupdatefound;
 };