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

media/audio/win/core_audio_util_win.h

Index: media/audio/win/core_audio_util_win.h
diff --git a/media/audio/win/core_audio_util_win.h b/media/audio/win/core_audio_util_win.h
index a210af906ea6e3366aaa1834d7497f572dc075e3..e78467ec3acf6cbef91f36e2426e95485517b544 100644
--- a/media/audio/win/core_audio_util_win.h
+++ b/media/audio/win/core_audio_util_win.h
@@ -37,6 +37,8 @@ class MEDIA_EXPORT CoreAudioUtil {
   // 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.