[PROTOTYPE] ServiceWorker: Add ServiceWorkerMessageEvent impl.

Source/modules/serviceworkers/ServiceWorkerMessageEvent.idl

+// Copyright 2015 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/#serviceworkermessage-event-section
+
+[
+    EventConstructor,
+    Exposed=(Window,Worker),
+    RuntimeEnabled=ServiceWorker,
+] interface ServiceWorkerMessageEvent : Event {
+    [InitializedByEventConstructor] readonly attribute DOMString origin;
+    [InitializedByEventConstructor] readonly attribute DOMString lastEventId;
+    [InitializedByEventConstructor] readonly attribute (ServiceWorker or MessagePort)? source;
+    [InitializedByEventConstructor, Custom=Getter] readonly attribute any data;
+    [InitializedByEventConstructor] readonly attribute MessagePort[] ports;
+    [Custom] void initServiceWorkerMessageEvent([Default=Undefined] optional DOMString typeArg,
+                                   [Default=Undefined] optional boolean canBubbleArg,
+                                   [Default=Undefined] optional boolean cancelableArg,
+                                   [Default=Undefined] optional any dataArg,
+                                   [Default=Undefined] optional DOMString originArg,
+                                   [Default=Undefined] optional DOMString lastEventIdArg,
+                                   [Default=Undefined] optional (ServiceWorker or MessagePort) sourceArg,
+                                   [Default=Undefined] optional MessagePort[] messagePorts);
+};