Add GetMatchingOutputDeviceID to CoreAudioUtil.

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 62 matching lines @@
   // the same controller so it is useful for doing things like determining
   // whether a set of output and input devices belong to the same controller.
   // The device enumerator is required as well as the device itself since
   // looking at the device topology is required and we need to open up
   // associated devices to determine the controller id.
   // If the ID could not be determined for some reason, an empty string is
   // returned.
   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);
 
   // Returns true if the provided unique |device_id| corresponds to the current
   // default device for the specified by a data-flow direction and role.
   static bool DeviceIsDefault(
       EDataFlow flow, ERole role, const std::string& device_id);
 
   // Query if the audio device is a rendering device or a capture device.
@@ skipping 101 matching lines @@
 
  private:
   CoreAudioUtil() {}
   ~CoreAudioUtil() {}
   DISALLOW_COPY_AND_ASSIGN(CoreAudioUtil);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_WIN_CORE_AUDIO_UTIL_WIN_H_