Add is_chromecast to GN build.

build/config/chromecast_build.gni

+# 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.
+
+# This block should only contain Chromecast-specific build arguments used by
+# by components outside of //chromecast.
+declare_args() {
+  # Set this true for a Chromecast build.

[brettw, 2015/09/17 22:52:14]

It would be good to document here the platforms that are supported for setting
this flag (I assume Linux only?) Also, an assert at the bottom of the file
(outside declare_args) like:
  assert(linux || !is_chromecast, "Chromecast requires Linux")
(or whatever the restriction is) would be helpful.

[slan, 2015/09/18 02:31:26]

Done. Internal builds support Linux and Android, public builds support Linux.

+  is_chromecast = false
+}