VideoPlayer: Create subtitles button.

ui/file_manager/video_player/css/media_controls.css

Index: ui/file_manager/video_player/css/media_controls.css
diff --git a/ui/file_manager/video_player/css/media_controls.css b/ui/file_manager/video_player/css/media_controls.css
index cf1e6cc9c3050c5199b503f1059c5adeccfe8d41..fd1acfd6ed9bc996811b8d90e0b12b840a7a2451 100644
--- a/ui/file_manager/video_player/css/media_controls.css
+++ b/ui/file_manager/video_player/css/media_controls.css
@@ -176,6 +176,20 @@ paper-slider.volume {
     url(../images/media/2x/media_fullscreen_exit.png) 2x);
 }
 
+.media-button.subtitles {
+  background-image: -webkit-image-set(
+    url(../images/media/media_subtitles.png) 1x,
+    url(../images/media/2x/media_subtitles.png) 2x);
+}
+
+.media-button.subtitles:not([showing]) {
+  opacity: 0.2;
+}
+
+.media-button.subtitles[unavailable] {
+  display: none;
+}
+
 .playback-state-icon {
   -webkit-animation: none;
   background-position: center center;