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

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 889f16ec24655dd6dd7e465d609c8ce131de8ac3..08fee526b47b94cfda5e28c5613832a1d0daa3a1 100644
--- a/ui/file_manager/video_player/css/media_controls.css
+++ b/ui/file_manager/video_player/css/media_controls.css
@@ -164,6 +164,12 @@ paper-slider.volume {
     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;