Add a simple, safe gain PostProcessor.

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

Index: chromecast/media/cma/backend/alsa/volume_map.h
diff --git a/chromecast/media/cma/backend/alsa/volume_map.h b/chromecast/media/cma/backend/alsa/volume_map.h
new file mode 100644
index 0000000000000000000000000000000000000000..493599dddc2607af578d2cffb4762e2c0344a5b4
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/volume_map.h
@@ -0,0 +1,40 @@
+// 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_VOLUME_MAP_H_
+#define CHROMECAST_MEDIA_CMA_BACKEND_ALSA_VOLUME_MAP_H_
+
+#include <vector>
+
+#include "base/macros.h"
+
+namespace chromecast {
+namespace media {
+
+class VolumeMap {
+ public:
+  VolumeMap();
+  ~VolumeMap();
+
+  float VolumeToDbFS(float volume);
+
+  float DbFSToVolume(float db);
+
+ private:
+  struct LevelToDb {
+    float level;
+    float db;
+  };
+
+  void UseDefaultVolumeMap();
+
+  std::vector<LevelToDb> volume_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(VolumeMap);
+};
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_BACKEND_ALSA_VOLUME_MAP_H_