[Extensions Bindings] Add a messaging test

chrome/test/data/extensions/api_test/native_bindings/content_script.js

+// Copyright 2017 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.
+
+chrome.runtime.onConnect.addListener((port) => {
+  port.onMessage.addListener((message) => {
+    chrome.test.assertEq('background page', message);
+    port.postMessage('content script');
+  });
+});
+
+chrome.runtime.sendMessage('startFlow', function(response) {
+  chrome.test.assertEq('started', response);
+});