Add a simple, safe gain PostProcessor.

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

Index: chromecast/media/cma/backend/alsa/post_processor_factory.h
diff --git a/chromecast/media/cma/backend/alsa/post_processor_factory.h b/chromecast/media/cma/backend/alsa/post_processor_factory.h
new file mode 100644
index 0000000000000000000000000000000000000000..b21352c6165d59b0bc8709398a9cb279add2df3d
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/post_processor_factory.h
@@ -0,0 +1,44 @@
+// 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_POST_PROCESSOR_FACTORY_H_
+#define CHROMECAST_MEDIA_CMA_BACKEND_ALSA_POST_PROCESSOR_FACTORY_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+
+namespace base {
+class ScopedNativeLibrary;
+}  // namespace base
+
+namespace chromecast {
+namespace media {
+
+class AudioPostProcessor;
+
+class PostProcessorFactory {
+ public:
+  PostProcessorFactory();
+  ~PostProcessorFactory();
+
+  std::unique_ptr<AudioPostProcessor> CreatePostProcessor(
+      const std::string& so_name,
+      const std::string& config,
+      int channels);
+
+ private:
+  // Contains all libraries in use;
+  // Functions in shared objects cannot be used once library is closed.
+  std::vector<std::unique_ptr<base::ScopedNativeLibrary>> libraries_;
+
+  DISALLOW_COPY_AND_ASSIGN(PostProcessorFactory);
+};
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_BACKEND_ALSA_POST_PROCESSOR_FACTORY_H_