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

ui/file_manager/video_player/js/video_player.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.
 
 /**
  * @param {!HTMLElement} playerContainer Main container.
  * @param {!HTMLElement} videoContainer Container for the video element.
  * @param {!HTMLElement} controlsContainer Container for video controls.
  * @constructor
  * @struct
@@ skipping 358 matching lines @@
               this.controls.attachMedia(this.videoElement_);
             }.bind(this));
           }.bind(this));
     } else {
       metrics.recordPlayType(metrics.PLAY_TYPE.LOCAL);
       videoPlayerElement.removeAttribute('casting');
 
       this.videoElement_ = document.createElement('video');
       getRequiredElement('video-container').appendChild(this.videoElement_);
 
+      var videoUrl = video.toURL();
       this.controls.attachMedia(this.videoElement_);
-      this.videoElement_.src = video.toURL();
+      var source = document.createElement('source');
+      source.src = videoUrl;
+      this.videoElement_.appendChild(source);
 
       media.isAvailableForCast().then(function(result) {
         if (result)
           videoPlayerElement.setAttribute('castable', true);
         else
           videoPlayerElement.removeAttribute('castable');
       }).catch(function() {
         videoPlayerElement.setAttribute('castable', true);
       });
 
-      videoElementInitializePromise = Promise.resolve();
+      videoElementInitializePromise = this.searchSubtitle_(videoUrl)
+          .then(function(subltitleUrl) {
+            if (subltitleUrl) {
+              var track = document.createElement('track');
+              track.src = subltitleUrl;
+              track.kind = 'subtitles';
+              track.default = true;
+              this.videoElement_.appendChild(track);
+            }
+          }.bind(this));
     }
-
     videoElementInitializePromise
         .then(function() {
           var handler = function(currentPos) {
             if (currentPos === this.currentPos_) {
               if (opt_callback)
                 opt_callback();
               videoPlayerElement.removeAttribute('loading');
             }
 
             this.videoElement_.removeEventListener('loadedmetadata', handler);
           }.wrap(this, this.currentPos_);
 
           this.videoElement_.addEventListener('loadedmetadata', handler);
 
           this.videoElement_.addEventListener('play', function() {
             chrome.power.requestKeepAwake('display');
             this.updateInactivityWatcherState_();
           }.wrap(this));
           this.videoElement_.addEventListener('pause', function() {
             chrome.power.releaseKeepAwake();
             this.updateInactivityWatcherState_();
           }.wrap(this));
-
-          this.videoElement_.load();
+          // TODO(ryoh):
+          // If you modify the video element that is already inserted,
+          // you have to call load() method.
+          // https://dev.w3.org/html5/spec-author-view/video.html
+          // But we always create new video element (see above),
+          // we don't have to call load().
+          // If you call load() method here,
+          // you can't see subtitles.
+          // (It might be a bug: https://crbug.com/594537)
+          //this.videoElement_.load();
           callback();
         }.bind(this))
         // In case of error.
         .catch(function(error) {
           if (this.currentCast_)
             metrics.recordCastVideoErrorAction();
 
           videoPlayerElement.removeAttribute('loading');
           console.error('Failed to initialize the video element.',
                         error.stack || error);
           this.controls_.showErrorMessage(
               'VIDEO_PLAYER_VIDEO_FILE_UNSUPPORTED');
           callback();
         }.bind(this));
   }.wrap(this));
 };
 
 /**
+ * Search subtile file corresponding to a video.
+ * @param {string} url a url of a video.
+ * @return {string} a url of subtitle file, or an empty string.
+ */
+VideoPlayer.prototype.searchSubtitle_ = function(url) {
+  var baseUrl = util.splitExtension(url)[0];
+  var resolveLocalFileSystemWithExtension = function(extension) {
+    return new Promise(
+        window.webkitResolveLocalFileSystemURL.bind(null, baseUrl + extension));
+  };
+  return resolveLocalFileSystemWithExtension('.vtt').then(function(subtitle) {
+    return subtitle.toURL();
+  }).catch(function() {
+    return '';
+  });
+};
+
+/**
  * Plays the first video.
  */
 VideoPlayer.prototype.playFirstVideo = function() {
   this.currentPos_ = 0;
   this.reloadCurrentVideo(this.onFirstVideoReady_.wrap(this));
 };
 
 /**
  * Unloads the current video.
  * @param {boolean=} opt_keepSession If true, keep using the current session.
@@ skipping 259 matching lines @@
   return new Promise(function(fulfill, reject) {
     util.URLsToEntries(window.appState.items, function(entries) {
       metrics.recordOpenVideoPlayerAction();
       metrics.recordNumberOfOpenedFiles(entries.length);
 
       player.prepare(entries);
       player.playFirstVideo(player, fulfill);
     }.wrap());
   }.wrap());
 }.wrap());