Add basic support for "googDucking" to getUserMedia on Windows.

media/audio/win/core_audio_util_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.
 
 // Utility methods for the Core Audio API on Windows.
 // Always ensure that Core Audio is supported before using these methods.
 // Use media::CoreAudioIsSupported() for this purpose.
 // Also, all methods must be called on a valid COM thread. This can be done
 // by using the base::win::ScopedCOMInitializer helper class.
 
@@ skipping 19 matching lines @@
 // Represents audio channel configuration constants as understood by Windows.
 // E.g. KSAUDIO_SPEAKER_MONO.  For a list of possible values see:
 // http://msdn.microsoft.com/en-us/library/windows/hardware/ff537083(v=vs.85).aspx
 typedef uint32 ChannelConfig;
 
 class MEDIA_EXPORT CoreAudioUtil {
  public:
   // Returns true if Windows Core Audio is supported.
   // Always verify that this method returns true before using any of the
   // methods in this class.
+  // WARNING: This function must be called once from the main thread before
+  // it is safe to call from other threads.
   static bool IsSupported();
 
   // Converts between reference time to base::TimeDelta.
   // One reference-time unit is 100 nanoseconds.
   // Example: double s = RefererenceTimeToTimeDelta(t).InMillisecondsF();
   static base::TimeDelta RefererenceTimeToTimeDelta(REFERENCE_TIME time);
 
   // Returns AUDCLNT_SHAREMODE_EXCLUSIVE if --enable-exclusive-mode is used
   // as command-line flag and AUDCLNT_SHAREMODE_SHARED otherwise (default).
   static AUDCLNT_SHAREMODE GetShareMode();
@@ skipping 174 matching lines @@
 
  private:
   CoreAudioUtil() {}
   ~CoreAudioUtil() {}
   DISALLOW_COPY_AND_ASSIGN(CoreAudioUtil);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_WIN_CORE_AUDIO_UTIL_WIN_H_