Turn audio ducking on by default on Windows again.

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::CoreAudioUtil::IsSupported() 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 182 matching lines @@
                                         EDataFlow data_flow);
 
   // After activating an IAudioClient interface on an audio endpoint device,
   // the client must initialize it once, and only once, to initialize the audio
   // stream between the client and the device. In shared mode, the client
   // connects indirectly through the audio engine which does the mixing.
   // In exclusive mode, the client connects directly to the audio hardware.
   // 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 if NULL is passed for |session_guid|, otherwise the client
+  // will be associated with the specified session.
   static HRESULT SharedModeInitialize(IAudioClient* client,
                                       const WAVEFORMATPCMEX* format,
                                       HANDLE event_handle,
-                                      uint32* endpoint_buffer_size);
+                                      uint32* endpoint_buffer_size,
+                                      const GUID* session_guid);
+
   // TODO(henrika): add ExclusiveModeInitialize(...)
 
   // Create an IAudioRenderClient client for an existing IAudioClient given by
   // |client|. The IAudioRenderClient interface enables a client to write
   // output data to a rendering endpoint buffer.
   static ScopedComPtr<IAudioRenderClient> CreateRenderClient(
       IAudioClient* client);
 
   // Create an IAudioCaptureClient client for an existing IAudioClient given by
   // |client|. The IAudioCaptureClient interface enables a client to read
   // input data from a capture endpoint buffer.
   static ScopedComPtr<IAudioCaptureClient> CreateCaptureClient(
       IAudioClient* client);
 
   // Fills up the endpoint rendering buffer with silence for an existing
   // IAudioClient given by |client| and a corresponding IAudioRenderClient
   // given by |render_client|.
   static bool FillRenderEndpointBufferWithSilence(
       IAudioClient* client, IAudioRenderClient* render_client);
 
  private:
   CoreAudioUtil() {}
   ~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 |SharedModeInitialize|.
+extern const GUID kCommunicationsSessionId;
+
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_WIN_CORE_AUDIO_UTIL_WIN_H_