Add is_chromecast to GN build.

ui/ozone/platform/cast/cast.gypi

 # 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.
 
 {
   'variables': {
     'cast_graphics_gyp%': '../../chromecast/chromecast.gyp',
     'libcast_media_gyp%': '../../chromecast/media/media.gyp',
     'internal_ozone_platform_deps': [
       'ozone_platform_cast',
     ],
     'internal_ozone_platform_unittest_deps': [ ],
     'internal_ozone_platforms': [
       'cast',
     ],
   },
   'targets': [
+    # GN target: //ui/ozone/platform/cast:cast
     {
       'target_name': 'ozone_platform_cast',
       'type': 'static_library',
       'dependencies': [
         '<(cast_graphics_gyp):libcast_graphics_1.0',
         '<(libcast_media_gyp):libcast_media_1.0',
         '../events/events.gyp:events',
         '../gfx/gfx.gyp:gfx',
         '../gfx/gfx.gyp:gfx_geometry',
         '../../base/base.gyp:base',
@@ skipping 15 matching lines @@
         'platform_window_cast.cc',
         'platform_window_cast.h',
         'surface_factory_cast.cc',
         'surface_factory_cast.h',
         'surface_ozone_egl_cast.cc',
         'surface_ozone_egl_cast.h',
       ],
     },
   ],
 }