[Chromoting] Add AudioVolumeApplier to reduce the complexity and the dependency of kChannels

remoting/host/audio_volume_filter.h

+// 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 REMOTING_HOST_AUDIO_VOLUME_FILTER_H_
+#define REMOTING_HOST_AUDIO_VOLUME_FILTER_H_
+
+#include "remoting/host/audio_silence_detector.h"
+
+namespace remoting {
+
+// A component to modify input audio sample to apply the audio level. This class
+// is used on platforms which returns non-adjusted audio samples, e.g. Windows.
+// This class supports frames with 16 bits per sample only.
+class AudioVolumeFilter {
+ public:
+  // See AudioSilenceDetector for the meaning of |silence_threshold|.
+  explicit AudioVolumeFilter(int silence_threshold);
+  virtual ~AudioVolumeFilter();
+
+  // Adjusts audio samples in |data|. If the samples are silent before applying
+  // the volume level or the GetAudioLevel() returns 0, this function returns
+  // false. If |frames| is 0, this function also returns false.
+  bool Apply(int16_t* data, size_t frames);
+
+  // Updates the sampling rate and channels.
+  void Initialize(int sampling_rate, int channels);
+
+ protected:
+  // Returns the volume level in [0, 1].

[Sergey Ulanov, 2017/05/23 00:02:14]

I think it's important to make it clear that this is a normalized scalar value,
i.e. sample values can be simply multiplied by the result of this function to
apply volume. Otherwise one might assume that it's a volume slider position,
which is different. 

[Hzj_jie, 2017/05/23 02:54:27]

Done.

+  virtual float GetAudioLevel() = 0;
+
+ private:
+  AudioSilenceDetector silence_detector_;
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_AUDIO_VOLUME_FILTER_H_