Add a test to cover service worker update codepath when extension is updated

chrome/test/data/extensions/api_test/service_worker/update/v2/background.js

+// 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.
+
+var controllerChangePromise = new Promise(function(resolve, reject) {
+  navigator.serviceWorker.oncontrollerchange = function(e) {
+    navigator.serviceWorker.ready.then(function(registration) {
+      resolve(registration.active);
+    }).catch(function(err) {
+      reject('oncontrollerchange failure');
+    });
+  };
+});
+
+controllerChangePromise.then(function(serviceWorker) {
+  var channel = new MessageChannel();
+  channel.port1.onmessage = function(e) {
+    chrome.test.log('Message received from SW: ' + e.data);
+    chrome.test.sendMessage(e.data);
+  };
+  serviceWorker.postMessage('ping', [channel.port2]);
+}).catch(function(err) {
+  console.log(err);
+  chrome.test.sendMessage('FAILURE_V2');
+});