Reland of land "media: Refactor MojoMediaClient" with fix.

chromecast/media/mojo/BUILD.gn

 # Copyright 2016 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("//media/media_options.gni")
 
 # We support mojo media service hosted in the browser process only.
 assert(enable_mojo_media == "browser")
 
 source_set("mojo") {
   sources = [
     "cast_mojo_media_client.cc",
+    "cast_mojo_media_client.h",
   ]
 
   public_deps = [
     "//media/mojo/services:application",
   ]
 }