Video Player: Add a message which is shown on casting

ui/file_manager/video_player/css/header.css

Index: ui/file_manager/video_player/css/header.css
diff --git a/ui/file_manager/video_player/css/header.css b/ui/file_manager/video_player/css/header.css
index 5a5b738717632e4bfeef2b11c831d9b0f3096292..fa37d554c901a962e94ec24b1b8c964e42e8a240 100644
--- a/ui/file_manager/video_player/css/header.css
+++ b/ui/file_manager/video_player/css/header.css
@@ -75,6 +75,12 @@
       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;
 }