Add is_chromecast to GN build.

chromecast/media/base/BUILD.gn

 # 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.
 
 import("//build/config/crypto.gni")
 import("//chromecast/chromecast.gni")
 
 declare_args() {
   libcast_media_target = ""
 }
 
+# TODO(slan): See if the dependency on //media can be broken.
 source_set("base") {
   sources = [
     "decrypt_context_impl.cc",
     "decrypt_context_impl.h",
     "decrypt_context_impl_clearkey.cc",
     "decrypt_context_impl_clearkey.h",
     "key_systems_common.cc",
     "key_systems_common.h",
     "media_caps.cc",
     "media_caps.h",
@@ skipping 29 matching lines @@
 
 shared_library("libcast_media_default") {
   output_name = "libcast_media_1.0"
 
   sources = [
     "cast_media_default.cc",
   ]
 
   configs += [ "//chromecast:config" ]
 
+  public_deps = [
+    "//chromecast/public",
+    "//chromecast/public/media",
+  ]
+
   deps = [
     "//build/config/sanitizers:deps",
     "//chromecast/media/cma/backend",
-    "//chromecast/public",
-    "//chromecast/public/media",
   ]
 }