[chromecast] Moves CastAudioOutputStream::Backend to CMA thread.

chromecast/media/cma/backend/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("//build/config/chromecast_build.gni")
 import("//chromecast/chromecast.gni")
 import("//media/media_options.gni")
 
 source_set("backend") {
   sources = [
-    "media_pipeline_backend_factory.cc",
     "media_pipeline_backend_factory.h",
+    "media_pipeline_backend_factory_impl.cc",
+    "media_pipeline_backend_factory_impl.h",
     "media_pipeline_backend_manager.cc",
     "media_pipeline_backend_manager.h",
     "media_pipeline_backend_wrapper.cc",
     "media_pipeline_backend_wrapper.h",
   ]
 
   public_deps = [
     "//chromecast/public",
     "//chromecast/public/media",
   ]
@@ skipping 82 matching lines @@
   sources = [
     "video_decoder_null.cc",
     "video_decoder_null.h",
   ]
 
   deps = [
     "//base",
     "//chromecast/public/media",
   ]
 }