Support external audio mixer.

webrtc/media/engine/nullwebrtcvideoengine_unittest.cc

Index: webrtc/media/engine/nullwebrtcvideoengine_unittest.cc
diff --git a/webrtc/media/engine/nullwebrtcvideoengine_unittest.cc b/webrtc/media/engine/nullwebrtcvideoengine_unittest.cc
index c63dbfb0109b623149fab5e2a4ab1549aef3f671..a8f2421610243413b927f907d7bdc0fbbdb84938 100644
--- a/webrtc/media/engine/nullwebrtcvideoengine_unittest.cc
+++ b/webrtc/media/engine/nullwebrtcvideoengine_unittest.cc
@@ -25,7 +25,9 @@ class WebRtcMediaEngineNullVideo
       WebRtcVideoEncoderFactory* video_encoder_factory,
       WebRtcVideoDecoderFactory* video_decoder_factory)
       : CompositeMediaEngine<WebRtcVoiceEngine, NullWebRtcVideoEngine>(
-            adm, audio_decoder_factory) {
+            adm,
+            audio_decoder_factory,
+            nullptr) {
     video_.SetExternalDecoderFactory(video_decoder_factory);
     video_.SetExternalEncoderFactory(video_encoder_factory);
   }