ServiceWorker: Use FrozenArray for *MessageEvent.ports.

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

 // Copyright 2016 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#extendablemessage-event-section
 
 [
     // TODO(bashi): Stop using CustomConstructor once we solve reference
     // circulation between Blink and V8. http://crbug.com/501866
     // Constructor should be:
     // Constructor(DOMString type, optional ExtendableMessageEventInit eventInitDict),
     CustomConstructor,
     Exposed=ServiceWorker,
 ] interface ExtendableMessageEvent : ExtendableEvent {
     [Custom=Getter] readonly attribute any data;
     readonly attribute DOMString origin;
     readonly attribute DOMString lastEventId;
     [SameObject] readonly attribute (Client or ServiceWorker or MessagePort)? source;
-    [SameObject] readonly attribute MessagePort[]? ports;
+    readonly attribute FrozenArray<MessagePort>? ports;
 };