Service Worker: Add a warning message for Client.postMessage

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#service-worker-client-interface
 [
     Exposed=ServiceWorker,
     RuntimeEnabled=ServiceWorker,
     GarbageCollected,
     TypeChecking=Interface,
     ImplementedAs=ServiceWorkerClient,
 ] interface Client {
     [RuntimeEnabled=ServiceWorkerClientAttributes] readonly attribute USVString url;
-    [Custom, RaisesException, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
+    // FIXME: Un-deprecate when the discussion on https://github.com/slightlyoff/ServiceWorker/issues/609 settles.
+    [Custom, RaisesException, DeprecateAs=ServiceWorkerClientPostMessage, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
 };