[Chromoting] Implement down mixing in AudioPump

remoting/host/audio_capturer_win.h

 // Copyright (c) 2012 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_CAPTURER_WIN_H_
 #define REMOTING_HOST_AUDIO_CAPTURER_WIN_H_
 
 #include <audioclient.h>
 #include <mmdeviceapi.h>
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/threading/thread_checker.h"
 #include "base/timer/timer.h"
 #include "base/win/scoped_co_mem.h"
 #include "base/win/scoped_comptr.h"
 #include "remoting/host/audio_capturer.h"
 #include "remoting/host/win/audio_volume_filter_win.h"
 #include "remoting/proto/audio.pb.h"
 
 namespace remoting {
 
 class DefaultAudioDeviceChangeDetector;
 
+// An AudioCapturer implementation for Windows by using Windows Audio Session
+// API, a.k.a. WASAPI. It supports up to 8 channels, but treats all layouts as
+// a most commonly used one. E.g. 3.1 and surround layouts will both be marked
+// as surround layout.
 class AudioCapturerWin : public AudioCapturer {
  public:
   AudioCapturerWin();
   ~AudioCapturerWin() override;
 
   // AudioCapturer interface.
   bool Start(const PacketCapturedCallback& callback) override;
 
  private:
   // Executes Deinitialize() and Initialize(). If Initialize() function call
@@ skipping 38 matching lines @@
   HRESULT last_capture_error_;
 
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioCapturerWin);
 };
 
 }  // namespace remoting
 
 #endif  // REMOTING_HOST_AUDIO_CAPTURER_WIN_H_