Video Player: Add a message which is shown on casting

ui/file_manager/video_player/css/header.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. */
 
 #video-player > .header {
   -webkit-app-region: drag;
   background: rgba(30, 30, 30, 0.8);
   border-bottom: 1px solid rgba(50, 50, 50, 0.8);
   display: flex;
   flex-direction: row;
@@ skipping 57 matching lines @@
   min-width: 32px;
   width: 32px;
 }
 
 #video-player > .header > button.cast-button {
   background-image: -webkit-image-set(
       url(../images/100/cast_off.png) 1x,
       url(../images/200/cast_off.png) 2x);
 }
 
+#video-player[casting] > .header > button.cast-button {
+  background-image: -webkit-image-set(
+      url(../images/100/cast_on.png) 1x,
+      url(../images/200/cast_on.png) 2x);
+}
+
 #video-player > .header > button.cast-button.hidden {
   display: none;
 }
 
 #video-player > .header > button.minimize-button {
   background-image: -webkit-image-set(
       url(chrome://resources/images/apps/topbar_button_minimize.png) 1x,
       url(chrome://resources/images/2x/apps/topbar_button_minimize.png) 2x);
 }
 
 #video-player > .header > button.maximize-button {
   background-image: -webkit-image-set(
       url(chrome://resources/images/apps/topbar_button_maximize.png) 1x,
       url(chrome://resources/images/2x/apps/topbar_button_maximize.png) 2x);
 }
 
 #video-player > .header > button.close-button {
   background-image: -webkit-image-set(
       url(chrome://resources/images/apps/topbar_button_close.png) 1x,
       url(chrome://resources/images/2x/apps/topbar_button_close.png) 2x);
 }