Service Worker: Include promise types in IDL

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
 [
     ActiveDOMObject,
     RuntimeEnabled=ServiceWorker,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface ServiceWorkerRegistration : EventTarget {
     [Unforgeable] readonly attribute ServiceWorker? installing;
     [Unforgeable] readonly attribute ServiceWorker? waiting;
     [Unforgeable] readonly attribute ServiceWorker? active;
+
     readonly attribute USVString scope;
 
-    [CallWith=ScriptState] Promise unregister();
+    [CallWith=ScriptState] Promise<boolean> unregister();
 
     attribute EventHandler onupdatefound;
 };