Add is_chromecast to GN build.

chromecast/build/args.gn

-# Build arguments for Chromecast.
-# Copy the contents of this file to gn args.
+# Copyright 2015 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.
 
+# Note: This file is being deprecated. Do not add new args here (b/23601081).
+
+# TODO(slan): Include the chromecast build tree as a dependency at root when
+# the GN build is stable.
 root_extra_deps = [ "//chromecast" ]
 
-# Enable the use of browser-side CDM.
-enable_browser_cdms = true
-
-# Enable plugins, but explicitly disable use of Pepper CDMs.
-enable_plugins = true
-enable_pepper_cdms = false
-
-# Flags set from build/common.gypi
-
-# Compile with MSE support for MPEG2 TS. See //media/media_options.gni for more
-# detail.
-enable_mpeg2ts_stream_parser = true
-
-# Instead of maintaining a separate "Chromecast" ffmpeg branding, "ChromeOS" is
-# used, as it sufficiently covers all the required codecs.
-ffmpeg_branding = "ChromeOS"
-
-# Setting this to true generates libeglplatform_shim.so.1 from
-# //ui/ozone/platform/egltest:egl_platform_x11.
-ozone_platform_ozonex = true
-
-# Chromecast must use proprietary codecs. Setting this to true defines the
-# USE_PROPRIETARY_CODECS macro and the proper EME_CODEC macros.
-proprietary_codecs = true
-
-# See //build/config/ui.gni. Notice that use_x11 = is_linux && !use_ozone.
-use_ozone = true
-# TODO(halliwell): look into supporting Cast Ozone with GN.
-# ozone_platform_cast = 1
-# TODO(gyp): Add support for blink_logging_always_on and enable it.
-# blink_logging_always_on = 1
+is_chromecast_build = true
+chromecast_branding = "public"