Add a simple, safe gain PostProcessor.

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

Index: chromecast/media/cma/backend/alsa/cast_audio_json.h
diff --git a/chromecast/media/cma/backend/alsa/cast_audio_json.h b/chromecast/media/cma/backend/alsa/cast_audio_json.h
new file mode 100644
index 0000000000000000000000000000000000000000..abfaacbafaa151432a102538c7a839ea7c5a6551
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/cast_audio_json.h
@@ -0,0 +1,16 @@
+// 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.
+
+#ifndef CHROMECAST_MEDIA_CMA_BACKEND_ALSA_CAST_AUDIO_JSON_H_
+#define CHROMECAST_MEDIA_CMA_BACKEND_ALSA_CAST_AUDIO_JSON_H_
+
+namespace chromecast {
+namespace media {
+
+extern const char kCastAudioJsonFilePath[];
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_BACKEND_ALSA_CAST_AUDIO_JSON_H_