Add skeleton implementation of ExtensionPushMessagingEventRouter.

chrome/test/data/extensions/api_test/push_messaging/event_dispatch.js

+// Copyright (c) 2012 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.
+
+function verifyDetails(details) {
+  chrome.test.assertEq(1, details.subchannelId);
+  chrome.test.assertEq("payload", details.payload);
+}
+
+function testEventDispatch() {
+  chrome.experimental.pushMessaging.onMessage.addListener(
+      chrome.test.callbackPass(verifyDetails));
+  chrome.test.sendMessage('ready');
+}
+
+chrome.test.runTests([testEventDispatch]);