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

mojo/services/media/core/interfaces/BUILD.gn

 # Copyright 2014 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/module_args/mojo.gni")
 import("$mojo_sdk_root/mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
     "media_demux.mojom",
+    "media_renderer.mojom",
     "media_type_converter.mojom",
     "seeking_reader.mojom",
     "timeline_controller.mojom",
   ]
 }