Add is_chromecast to GN build.

build/config/chromecast_build.gni

Index: build/config/chromecast_build.gni
diff --git a/build/config/chromecast_build.gni b/build/config/chromecast_build.gni
new file mode 100644
index 0000000000000000000000000000000000000000..3fac6094bc679d3f9336d13f8b8ac047654e2b51
--- /dev/null
+++ b/build/config/chromecast_build.gni
@@ -0,0 +1,15 @@
+# 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. Chromecast builds are supported on
+  # Linux and Android.
+  is_chromecast = false
+}
+
+# Assert that Chromecast is being built for a supported platform.
+assert(is_linux || is_android || !is_chromecast,
+       "Chromecast builds are not supported on $target_os")