Improve the declared IDL type for postMessage()'s transferables

Source/modules/serviceworkers/Client.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#client-interface
 [
     Exposed=ServiceWorker,
     RuntimeEnabled=ServiceWorker,
     WillBeGarbageCollected,
 ] interface Client {
     readonly attribute unsigned long id;
 
     // FIXME: Currently we think targetOrigin's probably not necessary, but
     // re-add it if the ongoing issue [1] concludes the other way.
     // [1] https://github.com/slightlyoff/ServiceWorker/issues/190
-    [Custom, RaisesException, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional MessagePort[] messagePorts);
+    [Custom, RaisesException, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
 };