Support external audio mixer in webrtc 2.

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 49b050c593d4ea40cde8e7b29caa41177ce49c73..17e79c6cc10297cd7cd3a79366bd02e2fc4a44e1 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;
@@ -45,6 +46,14 @@ class WebRtcMediaEngineFactory {
           audio_decoder_factory,
       WebRtcVideoEncoderFactory* video_encoder_factory,
       WebRtcVideoDecoderFactory* video_decoder_factory);
+
+  static MediaEngineInterface* Create(
+      webrtc::AudioDeviceModule* adm,
+      const rtc::scoped_refptr<webrtc::AudioDecoderFactory>&
+          audio_decoder_factory,
+      WebRtcVideoEncoderFactory* video_encoder_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