VideoPlayer: Show fullscreen-exit button when the player window is fullscreened.

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;
@@ skipping 146 matching lines @@
 
 /* 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 {
+  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;
   height: 32px;
   left: 50%;
   margin-left: -16px;
   margin-top: -16px;
   opacity: 0;
@@ skipping 67 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);
 }