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

chromecast/media/cma/backend/media_pipeline_backend_factory.cc

-// Copyright 2017 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.
-
-#include "chromecast/media/cma/backend/media_pipeline_backend_factory.h"
-
-#include "chromecast/media/cma/backend/media_pipeline_backend_manager.h"
-#include "chromecast/public/media/media_pipeline_backend.h"
-#include "chromecast/public/media/media_pipeline_device_params.h"
-
-namespace chromecast {
-namespace media {
-
-MediaPipelineBackendFactory::MediaPipelineBackendFactory(
-    MediaPipelineBackendManager* media_pipeline_backend_manager)
-    : media_pipeline_backend_manager_(media_pipeline_backend_manager) {
-  DCHECK(media_pipeline_backend_manager_);
-}
-
-MediaPipelineBackendFactory::~MediaPipelineBackendFactory() {}
-
-std::unique_ptr<MediaPipelineBackend>
-MediaPipelineBackendFactory::CreateBackend(
-    const MediaPipelineDeviceParams& params) {
-  return media_pipeline_backend_manager_->CreateMediaPipelineBackend(params);
-}
-
-}  // namespace media
-}  // namespace chromecast