Allow content embedders to create AudioManager.

chromecast/media/audio/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.
 
 source_set("audio") {
   sources = [
     "cast_audio_manager.cc",
     "cast_audio_manager.h",
-    "cast_audio_manager_factory.cc",
-    "cast_audio_manager_factory.h",
     "cast_audio_output_stream.cc",
     "cast_audio_output_stream.h",
   ]
 
   deps = [
     "//base",
     "//chromecast/base",
     "//chromecast/media/base",
     "//chromecast/media/cma/backend",
     "//chromecast/media/cma/base",
     "//chromecast/public/media",
     "//media",
     "//media:shared_memory_support",
   ]
 }