Motown: Define MediaRenderer and make it the way we identify renderers.

examples/media_test/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("//mojo/public/mojo_application.gni")
 
 mojo_native_application("media_test") {
   sources = [
     "keystroke.cc",
     "keystroke.h",
     "media_test.cc",
     "media_test.h",
     "media_test_app.cc",
   ]
 
   deps = [
     "//base",
     "//mojo/application",
     "//mojo/environment:chromium",
     "//mojo/gpu",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/public/interfaces/application",
+    "//mojo/services/media/audio/interfaces",
     "//mojo/services/media/common/cpp",
     "//mojo/services/media/common/interfaces",
     "//mojo/services/media/control/interfaces",
   ]
 }