Turn audio ducking on by default on Windows again.

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 8727f97b51cd6c76be4e0a74c7c847f98b5fcc52..28eded171dc55b6ce4852af24609ef363731bec2 100644
--- a/media/audio/win/core_audio_util_win.h
+++ b/media/audio/win/core_audio_util_win.h
@@ -200,10 +200,21 @@ class MEDIA_EXPORT CoreAudioUtil {
   // If a valid event is provided in |event_handle|, the client will be
   // initialized for event-driven buffer handling. If |event_handle| is set to
   // NULL, event-driven buffer handling is not utilized.
+  // This function will initialize the audio client as part of the default
+  // audio session.
   static HRESULT SharedModeInitialize(IAudioClient* client,

[no longer working on chromium, 2014/07/08 10:42:35]

it seems that this method is not used in production code any more. Consider
removing it? or add a TODO to remove it?

[tommi (sloooow) - chröme, 2014/07/08 12:30:56]

I removed it and renamed the other one to SharedModeInitialize since there's
only one such function.

                                       const WAVEFORMATPCMEX* format,
                                       HANDLE event_handle,
                                       uint32* endpoint_buffer_size);
+
+  // Same as SharedModeInitialize with the addition of being able to specify
+  // a specific audio session to initialize the audio client as part of.
+  // NULL can be passed for |session_guid| which represents the default session.
+  static HRESULT SharedModeInitializeWithSession(IAudioClient* client,
+                                                 const WAVEFORMATPCMEX* format,
+                                                 HANDLE event_handle,
+                                                 uint32* endpoint_buffer_size,
+                                                 const GUID* session_guid);
   // TODO(henrika): add ExclusiveModeInitialize(...)
 
   // Create an IAudioRenderClient client for an existing IAudioClient given by
@@ -230,6 +241,13 @@ class MEDIA_EXPORT CoreAudioUtil {
   DISALLOW_COPY_AND_ASSIGN(CoreAudioUtil);
 };
 
+// The special audio session identifier we use when opening up the default
+// communication device.  This has the effect that a separate volume control
+// will be shown in the system's volume mixer and control over ducking and
+// visually observing the behavior of ducking, is easier.
+// Use with |SharedModeInitializeWithSession|.
+extern const GUID kCommunicationsSessionId;
+
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_WIN_CORE_AUDIO_UTIL_WIN_H_