Support external audio mixer.

webrtc/media/engine/webrtcvoiceengine.h

Index: webrtc/media/engine/webrtcvoiceengine.h
diff --git a/webrtc/media/engine/webrtcvoiceengine.h b/webrtc/media/engine/webrtcvoiceengine.h
index 03b493744b857f903cbb09f530794787466e7b8f..89fbf47f99eb80d1d57d593046b5fac3714a49f8 100644
--- a/webrtc/media/engine/webrtcvoiceengine.h
+++ b/webrtc/media/engine/webrtcvoiceengine.h
@@ -16,6 +16,7 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/api/audio/audio_mixer.h"
 #include "webrtc/api/call/audio_state.h"
 #include "webrtc/base/buffer.h"
 #include "webrtc/base/constructormagic.h"
@@ -48,11 +49,13 @@ class WebRtcVoiceEngine final : public webrtc::TraceCallback  {
 
   WebRtcVoiceEngine(
       webrtc::AudioDeviceModule* adm,
-      const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory);
+      const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory,
+      rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer);
   // Dependency injection for testing.
   WebRtcVoiceEngine(
       webrtc::AudioDeviceModule* adm,
       const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory,
+      rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer,
       VoEWrapper* voe_wrapper);
   ~WebRtcVoiceEngine() override;