Video Player: Show a checkmark on the currently playing device in cast menu

ui/file_manager/video_player/css/cast_menu.css

 /* 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. */
 
 .cast-menu {
   border: 1px solid #7f7f7f;
   border-radius: 1px;
   min-width: 120px;
   padding: 0;
   position: absolute;
   z-index: 1000;
 }
 
 .cast-menu.hidden {
   display: none;
 }
 
 .cast-menu > :not(hr) {
+  -webkit-padding-end: 14px;
+  -webkit-padding-start: 19px;
   background-color: #fff;
   font-size: 12px;
-  padding: 5px 10px; /* top&bottom, left&right */
+  padding-bottom: 5px;
+  padding-top: 5px;
   text-overflow: ellipsis;
 }
 
+.cast-menu > [checked]:not(hr) {
+  /* A checkmark has 19 px width. */
+  -webkit-padding-start: 0;
+}
+
 .cast-menu > :not(hr):hover {
   background-color: #eee;
 }