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

ui/file_manager/video_player/js/video_player.js

Index: ui/file_manager/video_player/js/video_player.js
diff --git a/ui/file_manager/video_player/js/video_player.js b/ui/file_manager/video_player/js/video_player.js
index bed41ba51340d46f09cffffeb3e3bf4c4cd10e94..a070db4a4cba29945c7c0139b9cc91f5ce4419d3 100644
--- a/ui/file_manager/video_player/js/video_player.js
+++ b/ui/file_manager/video_player/js/video_player.js
@@ -376,8 +376,11 @@ VideoPlayer.prototype.loadVideo_ = function(video, opt_callback) {
       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)
@@ -388,9 +391,18 @@ VideoPlayer.prototype.loadVideo_ = function(video, opt_callback) {
         videoPlayerElement.setAttribute('castable', true);
       });
 
-      videoElementInitializePromise = Promise.resolve();
+      videoElementInitializePromise = this.loadSubtitles_(videoUrl)
+          .then(function(subltitleUrl) {
+            if (subltitleUrl) {
+              var track = document.createElement('track');
+              track.src = subltitleUrl;
+              track.kind = 'subtitles';
+              track.default = true;
+              this.videoElement_.appendChild(track);
+            }
+            return Promise.resolve();

[yoshiki, 2016/03/14 04:10:52]

nit: returning resolve is unnecessary. please remove.

[ryoh, 2016/03/14 13:45:25]

Done.

+          }.bind(this));
     }
-
     videoElementInitializePromise
         .then(function() {
           var handler = function(currentPos) {
@@ -414,7 +426,6 @@ VideoPlayer.prototype.loadVideo_ = function(video, opt_callback) {
             this.updateInactivityWatcherState_();
           }.wrap(this));
 
-          this.videoElement_.load();

[ryoh, 2016/03/11 15:11:58]

Why do we need this line?
It seems 'loadedmetadata' event will be fired whether we call this function or
not.
If you call this function here, we can't see subtitles.
(I think it might be a bug of blink)

[yoshiki, 2016/03/14 04:10:52]

According to the spec, we need to call load() unless 'src' attribute on
'video'-tag element is modified.
https://dev.w3.org/html5/spec-author-view/video.html
So, I think we should do this.

However, even if the video player works correctly without this, you may remove
it but please leave the comment.

And if you think it's a bug, please file a bug to http://crbug.com/new. Thanks.

[ryoh, 2016/03/14 13:45:25]

Yeah, if we **modified** src attribute, you have to call load(), but we always
create new 'video' element (IIRC).
Maybe we don't have to call load().

OK, I reported this as a bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=594537

           callback();
         }.bind(this))
         // In case of error.
@@ -432,6 +443,19 @@ VideoPlayer.prototype.loadVideo_ = function(video, opt_callback) {
   }.wrap(this));
 };
 
+VideoPlayer.prototype.loadSubtitles_ = function(url) {

[yoshiki, 2016/03/14 04:10:52]

nit: Please write a JSdoc (short description of the method and parameter).

[ryoh, 2016/03/14 13:45:25]

Done.

+  var baseUrl = util.splitExtension(url)[0];
+  var resolveLocalFileSystem = function(extension) {

[yoshiki, 2016/03/14 04:10:52]

"resolveLocalFileSystem" looks confusing with resolveLocalFileSystemURL().
"resolveLocalFileSystemWIthExtension" might be better?

[ryoh, 2016/03/14 13:45:25]

Sounds nice! Thank you.

+    return new Promise(
+        window.webkitResolveLocalFileSystemURL.bind(null, baseUrl + extension));
+  }

[yoshiki, 2016/03/14 04:10:52]

nit: semicolon

[ryoh, 2016/03/14 13:45:25]

Done.

+  return resolveLocalFileSystem('.vtt').then(function(subtitle) {
+    return subtitle.toURL();
+  }).catch(function() {
+    return '';
+  });
+};
+
 /**
  * Plays the first video.
  */