Add GetMatchingOutputDeviceID to CoreAudioUtil.

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 154a33a8a4bef19b6c1eaab6b9f1a0abef2c5916..beb0e9c0c9b2296b21776cdda1a7aa92249c908e 100644
--- a/media/audio/win/core_audio_util_win.h
+++ b/media/audio/win/core_audio_util_win.h
@@ -80,6 +80,12 @@ class MEDIA_EXPORT CoreAudioUtil {
   static std::string GetAudioControllerID(IMMDevice* device,
       IMMDeviceEnumerator* enumerator);
 
+  // Accepts an id of an input device and finds a matching output device id.
+  // If the associated hardware does not have an audio output device (e.g.
+  // a webcam with a mic), an empty string is returned.
+  static std::string GetMatchingOutputDeviceID(
+      const std::string& input_device_id);
+
   // Gets the user-friendly name of the endpoint device which is represented
   // by a unique id in |device_id|.
   static std::string GetFriendlyName(const std::string& device_id);