Turn audio ducking on by default on Windows again.

media/audio/win/core_audio_util_win.cc

Index: media/audio/win/core_audio_util_win.cc
diff --git a/media/audio/win/core_audio_util_win.cc b/media/audio/win/core_audio_util_win.cc
index 71e8d717f62f4ea2fb164167487a5efd35676644..8743f902082ec0abd47a42456dd4748d24cf436c 100644
--- a/media/audio/win/core_audio_util_win.cc
+++ b/media/audio/win/core_audio_util_win.cc
@@ -23,6 +23,12 @@ using base::win::ScopedHandle;
 
 namespace media {
 
+// See header file for documentation.
+// {BE39AF4F-087C-423F-9303-234EC1E5B8EE}
+const GUID kCommunicationsSessionId = {
+  0xbe39af4f, 0x87c, 0x423f, { 0x93, 0x3, 0x23, 0x4e, 0xc1, 0xe5, 0xb8, 0xee }
+};
+
 enum { KSAUDIO_SPEAKER_UNSUPPORTED = 0 };
 
 // Converts Microsoft's channel configuration to ChannelLayout.
@@ -735,6 +741,13 @@ HRESULT CoreAudioUtil::SharedModeInitialize(IAudioClient* client,
                                             const WAVEFORMATPCMEX* format,
                                             HANDLE event_handle,
                                             uint32* endpoint_buffer_size) {
+  return SharedModeInitializeWithSession(
+      client, format, event_handle, endpoint_buffer_size, NULL);
+}
+
+HRESULT CoreAudioUtil::SharedModeInitializeWithSession(
+    IAudioClient* client, const WAVEFORMATPCMEX* format, HANDLE event_handle,
+    uint32* endpoint_buffer_size, const GUID* session_guid) {
   DCHECK(IsSupported());
 
   // Use default flags (i.e, dont set AUDCLNT_STREAMFLAGS_NOPERSIST) to
@@ -760,7 +773,7 @@ HRESULT CoreAudioUtil::SharedModeInitialize(IAudioClient* client,
                                   0,
                                   0,
                                   reinterpret_cast<const WAVEFORMATEX*>(format),
-                                  NULL);
+                                  session_guid);
   if (FAILED(hr)) {
     DVLOG(1) << "IAudioClient::Initialize: " << std::hex << hr;
     return hr;