Media Remoting: Add browser tests.

chrome/test/data/extensions/api_test/media_remoting/toggle_renderer.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.
+
+'use strict';
+
+let video = document.getElementById("video");
+video.onended = function () {
+  chrome.test.succeed();
+}
+
+chrome.test.runTests([
+  function startRemoting() {
+    mrTest.StartMediaRouter()
+    .then(result => {
+      const mediaRouterService = result['mrService'];
+      mediaRouterService.setHandlers(
+          new mrTest.ProviderManager(mediaRouterService,
+                                     new mrTest.Parameters()));
+    });
+    chrome.test.notifyPass();
+  }
+]);
+
+
+
+