Video Player: Check Cast extension before launching cast feature

ui/file_manager/video_player/js/video_player_scripts.js

 // 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.
 
 // The include directives are put into Javascript-style comments to prevent
 // parsing errors in non-flattened mode. The flattener still sees them.
 // Note that this makes the flattener to comment out the first line of the
 // included file but that's all right since any javascript file should start
 // with a copyright comment anyway.
 
@@ skipping 14 matching lines @@
 //<include src="../../../webui/resources/js/cr/ui/context_menu_handler.js"/>
 
 (function() {
 'use strict';
 
 //<include src="../../file_manager/common/js/async_util.js"/>
 //<include src="../../file_manager/common/js/util.js"/>
 //<include src="../../file_manager/foreground/js/media/media_controls.js"/>
 //<include src="../../file_manager/foreground/js/media/mouse_inactivity_watcher.js"/>
 
+//<include src="cast/cast_extension_discoverer.js"/>
 //<include src="cast/cast_video_element.js"/>
 //<include src="cast/media_manager.js"/>
 //<include src="cast/caster.js"/>
 
 //<include src="video_player.js"/>
 
 window.unload = unload;
 
 })();