ServiceWorker: Add initial implementation of ExtendableMessageEvent behind a flag

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
+
+[
+    Constructor(DOMString type, optional ExtendableMessageEventInit eventInitDict),
+    Exposed=ServiceWorker,
+    RuntimeEnabled=ServiceWorkerExtendableMessageEvent,
+] interface ExtendableMessageEvent : ExtendableEvent {
+    // TODO(nhiroki): Implement these attributes (crbug.com/543198).
+    // 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;
+};