Add is_chromecast to GN build.

chromecast/media/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("//chromecast/chromecast.gni")
 import("//testing/test.gni")
 
 group("media") {
   deps = [
     "//chromecast/media/audio",
     "//chromecast/media/base",
     "//chromecast/media/cdm",
     "//chromecast/media/cma",
   ]
 }
@@ skipping 37 matching lines @@
     "//chromecast/base/metrics:test_support",
     "//chromecast/public",
     "//media",
     "//media/base:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/gfx/geometry",
   ]
 }
 
+group("libcast_media") {
+  if (use_default_cast_media) {
+    public_deps = [
+      "//chromecast/media/base:libcast_media_default",
+    ]
+  } else {
+    public_deps = [
+      "$libcast_media_path:libcast_media_1.0",
+    ]
+  }
+}
+
 # TODO(slan): delete this target once Chromecast M44/earlier is obsolete.
 # See: b/21639416
 shared_library("libffmpegsumo") {
   output_name = "libffmpegsumo"
 }