Fix accessibility issues in AudioPlayer and VideoPlayer.

ui/file_manager/video_player/css/media_controls.css

 /* Copyright (c) 2012 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. */
 
 /* Common styles for media buttons. */
 
 .media-button {
   background-position: center;
   background-repeat: no-repeat;
   flex: none;
   height: 32px;
   position: relative;
   width: 32px;
 }
 
-.media-button:hover {
-  background-color: rgba(153, 153, 153, 0.2);
-}
-
-.media-button:active {
-  background-color: rgba(153, 153, 153, 0.4);
-}
-
 .media-button.disabled {
   background-color: transparent;
   opacity: 0.26;
 }
 
 .media-button.disabled,
 paper-slider[disabled],
 paper-slider.readonly {
   pointer-events: none;
 }
@@ skipping 101 matching lines @@
   pointer-events: auto;
 }
 
 /* Cast button. */
 
 .media-button.cast {
   background-image: -webkit-image-set(
     url(../images/media/media_chromecast.png) 1x,
     url(../images/media/2x/media_chromecast.png) 2x);
   display: none;
+  border-radius: 2px;
+}
+
+/* Reset browser's button style. */
+.media-button.cast {
+  background-color: transparent;
+  border: none;
+  cursor: pointer;
+  outline: none;
 }
 
 #video-player[cast-available][castable] .media-button.cast {
   display: block;
 }
 
 #video-player[casting] .media-button.cast {
   background-image: -webkit-image-set(
     url(../images/media/media_chromecast_casting.png) 1x,
     url(../images/media/2x/media_chromecast_casting.png) 2x);
 }
 
+.media-button.cast:focus:not(.using-mouse) {
+  box-shadow: 0 0 0 1px rgba(66, 133, 244, 0.5);
+}
+
 /* Fullscreen button. */
 /* There is no final decision whether we need a separate icon when toggled. */
 
 .media-button.fullscreen {
   background-image: -webkit-image-set(
     url(../images/media/media_fullscreen.png) 1x,
     url(../images/media/2x/media_fullscreen.png) 2x);
 }
 
-#video-player[fullscreen] .media-button.fullscreen {
+#controls[fullscreen] .media-button.fullscreen {
   background-image: -webkit-image-set(
     url(../images/media/media_fullscreen_exit.png) 1x,
     url(../images/media/2x/media_fullscreen_exit.png) 2x);
 }
 
 .playback-state-icon {
   -webkit-animation: none;
   background-position: center center;
   background-repeat: no-repeat;
   display: none;
@@ skipping 72 matching lines @@
     url(../images/media/media_play_onscreen.png) 1x,
     url(../images/media/2x/media_play_onscreen.png) 2x);
 }
 
 .playback-state-icon[state='pause'] {
   -webkit-animation: blowup 500ms;
   background-image: -webkit-image-set(
     url(../images/media/media_pause_onscreen.png) 1x,
     url(../images/media/2x/media_pause_onscreen.png) 2x);
 }