Add AudioManager::GetAudioOutputDeviceNames and implement for pulseaudio.

media/audio/audio_manager.h

Index: media/audio/audio_manager.h
diff --git a/media/audio/audio_manager.h b/media/audio/audio_manager.h
index 433b515f349b9b938f0c81ebf06c4bbcc5f2af88..ccce6a0a2f79a442eae24130177faa37f9d675f3 100644
--- a/media/audio/audio_manager.h
+++ b/media/audio/audio_manager.h
@@ -64,6 +64,10 @@ class MEDIA_EXPORT AudioManager {
   // recording.
   virtual void GetAudioInputDeviceNames(AudioDeviceNames* device_names) = 0;
 
+  // Appends a list of available output devices to |device_names|,
+  // which must initially be empty.
+  virtual void GetAudioOutputDeviceNames(AudioDeviceNames* device_names) = 0;
+
   // Factory for all the supported stream formats. |params| defines parameters
   // of the audio stream to be created.
   //