Support external audio mixer.

webrtc/media/engine/webrtcmediaengine.h

Index: webrtc/media/engine/webrtcmediaengine.h
diff --git a/webrtc/media/engine/webrtcmediaengine.h b/webrtc/media/engine/webrtcmediaengine.h
index dae63477d678ca728b79a091ef65d2d5e76c0a60..9aaebe7795f3c6837ebfb8e2aca7fdc6316cd55d 100644
--- a/webrtc/media/engine/webrtcmediaengine.h
+++ b/webrtc/media/engine/webrtcmediaengine.h
@@ -14,6 +14,7 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/api/audio/audio_mixer.h"
 #include "webrtc/config.h"
 #include "webrtc/media/base/mediaengine.h"
 
@@ -36,14 +37,16 @@ class WebRtcMediaEngineFactory {
   static MediaEngineInterface* Create(
       webrtc::AudioDeviceModule* adm,
       WebRtcVideoEncoderFactory* video_encoder_factory,
-      WebRtcVideoDecoderFactory* video_decoder_factory);
+      WebRtcVideoDecoderFactory* video_decoder_factory,
+      rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer);
 
   static MediaEngineInterface* Create(
       webrtc::AudioDeviceModule* adm,
       const rtc::scoped_refptr<webrtc::AudioDecoderFactory>&
           audio_decoder_factory,
       WebRtcVideoEncoderFactory* video_encoder_factory,
-      WebRtcVideoDecoderFactory* video_decoder_factory);
+      WebRtcVideoDecoderFactory* video_decoder_factory,
+      rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer);
 };
 
 // Verify that extension IDs are within 1-byte extension range and are not