Add subtitles track if there is a subtitles file with the same name.

ui/file_manager/video_player/js/test_util.js

 // Copyright 2014 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.
 
 /**
  * Returns if a video element playing the specified file meet the condition
  * which is given by a parameter.
  * @param {string} filename Name of video file to be checked. This must be same
  *     as entry.name() of the video file.
  * @param {function(!HTMLElement):boolean} testFunction
  */
 function testElement(filename, testFunction) {
   for (var appId in window.background.appWindows) {
     var contentWindow = window.background.appWindows[appId].contentWindow;
     if (contentWindow &&
         contentWindow.document.title === filename) {
-      var element = contentWindow.document.querySelector('video[src]');
+      var element = contentWindow.document.querySelector('video');
       if (element && testFunction(element))
         return true;
     }
   }
   return false;
 }
 
 /**
  * Returns if the specified file is being played.
  *
@@ skipping 35 matching lines @@
  * @param {!Array<string>} urls URLs to be opened.
  * @param {function(string)} callback Completion callback with the new window's
  *     App ID.
  */
 test.util.async.openVideoPlayer = function(urls, callback) {
   openVideoPlayerWindow(urls).then(callback);
 };
 
 // Register the test utils.
 test.util.registerRemoteTestUtils();