Add a simple, safe gain PostProcessor.

chromecast/media/cma/backend/alsa/cast_audio_json.cc

Index: chromecast/media/cma/backend/alsa/cast_audio_json.cc
diff --git a/chromecast/media/cma/backend/alsa/cast_audio_json.cc b/chromecast/media/cma/backend/alsa/cast_audio_json.cc
new file mode 100644
index 0000000000000000000000000000000000000000..57ad7bb0aec541725a6299cb826a9bf74ee31b0a
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/cast_audio_json.cc
@@ -0,0 +1,13 @@
+// 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/alsa/cast_audio_json.h"
+
+namespace chromecast {
+namespace media {
+
+const char kCastAudioJsonFilePath[] = "/etc/cast_audio.json";
+
+}  // namespace media
+}  // namespace chromecast