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..45961d6f87e7b5af6790af9fa3c47f4383301ba9 100644
--- a/webrtc/media/engine/webrtcmediaengine.h
+++ b/webrtc/media/engine/webrtcmediaengine.h
@@ -20,6 +20,7 @@
 namespace webrtc {
 class AudioDecoderFactory;
 class AudioDeviceModule;
+class AudioMixer;
 }
 namespace cricket {
 class WebRtcVideoDecoderFactory;
@@ -43,7 +44,8 @@ class WebRtcMediaEngineFactory {
       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