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..380beaa967bcb506df60484daf7b701e398205d6
--- /dev/null
+++ b/build/config/chromecast_build.gni
@@ -0,0 +1,10 @@
+# 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.
+  is_chromecast = false
+}