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

chromecast/media/cma/test/mock_media_pipeline_backend_factory.cc

Index: chromecast/media/cma/test/mock_media_pipeline_backend_factory.cc
diff --git a/chromecast/media/cma/test/mock_media_pipeline_backend_factory.cc b/chromecast/media/cma/test/mock_media_pipeline_backend_factory.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9f4d86a403905d5d7d48d886518255d81e478f19
--- /dev/null
+++ b/chromecast/media/cma/test/mock_media_pipeline_backend_factory.cc
@@ -0,0 +1,14 @@
+// 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/test/mock_media_pipeline_backend_factory.h"
+
+namespace chromecast {
+namespace media {
+
+MockMediaPipelineBackendFactory::MockMediaPipelineBackendFactory() = default;
+MockMediaPipelineBackendFactory::~MockMediaPipelineBackendFactory() = default;
+
+}  // namespace media
+}  // namespace chromecast