Add is_chromecast to GN build.

chromecast/media/BUILD.gn

Index: chromecast/media/BUILD.gn
diff --git a/chromecast/media/BUILD.gn b/chromecast/media/BUILD.gn
index cd72d9dbb47b8f7fa258ddeabb24e912067752f1..a5471ebb53a2ab6bd81e4f1d927e8777ed347b35 100644
--- a/chromecast/media/BUILD.gn
+++ b/chromecast/media/BUILD.gn
@@ -2,6 +2,7 @@
 # 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") {
@@ -59,6 +60,18 @@ test("cast_media_unittests") {
   ]
 }
 
+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 # TODO(slan): delete this target once Chromecast M44/earlier is obsolete.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") {