[Chromoting] Implement down mixing in AudioPump

remoting/host/audio_capturer_win.h

Index: remoting/host/audio_capturer_win.h
diff --git a/remoting/host/audio_capturer_win.h b/remoting/host/audio_capturer_win.h
index 0caca6324e5141a352b9eeb188d2bb2c30dca5f8..80e5b3d195fa13bc0802a89af8e0e853cff9a856 100644
--- a/remoting/host/audio_capturer_win.h
+++ b/remoting/host/audio_capturer_win.h
@@ -23,6 +23,10 @@ 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();