Support external audio mixer in webrtc.

An external audio mixer will be passed from PeerConnectionFactory to
AudioTransportProxy.

BUG=webrtc:6457

Review-Url: https://codereview.webrtc.org/2539213003
Cr-Commit-Position: refs/heads/master@{#15556}
diff --git a/webrtc/media/engine/webrtcmediaengine.h b/webrtc/media/engine/webrtcmediaengine.h
index 49b050c..3cca48f 100644
--- a/webrtc/media/engine/webrtcmediaengine.h
+++ b/webrtc/media/engine/webrtcmediaengine.h
@@ -21,6 +21,7 @@
 namespace webrtc {
 class AudioDecoderFactory;
 class AudioDeviceModule;
+class AudioMixer;
 }
 namespace cricket {
 class WebRtcVideoDecoderFactory;
@@ -44,7 +45,8 @@
       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