Support external audio mixer in webrtc 2.

webrtc/media/engine/webrtcvoiceengine.cc

Index: webrtc/media/engine/webrtcvoiceengine.cc
diff --git a/webrtc/media/engine/webrtcvoiceengine.cc b/webrtc/media/engine/webrtcvoiceengine.cc
index 2a0583c90944a9d2378896878b401f30a0b02c73..a36f3287057bc440a75eddf41cc1ed3dec206c5e 100644
--- a/webrtc/media/engine/webrtcvoiceengine.cc
+++ b/webrtc/media/engine/webrtcvoiceengine.cc
@@ -277,10 +277,16 @@ void GetOpusConfig(const AudioCodec& codec,
   voe_codec->rate = GetOpusBitrate(codec, *max_playback_rate);
 }
 
-webrtc::AudioState::Config MakeAudioStateConfig(VoEWrapper* voe_wrapper) {
+webrtc::AudioState::Config MakeAudioStateConfig(
+    VoEWrapper* voe_wrapper,
+    rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer) {
   webrtc::AudioState::Config config;
   config.voice_engine = voe_wrapper->engine();
-  config.audio_mixer = webrtc::AudioMixerImpl::Create();
+  if (audio_mixer) {
+    config.audio_mixer = audio_mixer;
+  } else {
+    config.audio_mixer = webrtc::AudioMixerImpl::Create();
+  }
   return config;
 }
 
@@ -540,14 +546,17 @@ bool WebRtcVoiceEngine::ToCodecInst(const AudioCodec& in,
 
 WebRtcVoiceEngine::WebRtcVoiceEngine(
     webrtc::AudioDeviceModule* adm,
-    const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory)
-    : WebRtcVoiceEngine(adm, decoder_factory, new VoEWrapper()) {
-  audio_state_ = webrtc::AudioState::Create(MakeAudioStateConfig(voe()));
+    const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory,
+    rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer)
+    : WebRtcVoiceEngine(adm, decoder_factory, audio_mixer, new VoEWrapper()) {
+  audio_state_ =
+      webrtc::AudioState::Create(MakeAudioStateConfig(voe(), audio_mixer));
 }
 
 WebRtcVoiceEngine::WebRtcVoiceEngine(
     webrtc::AudioDeviceModule* adm,
     const rtc::scoped_refptr<webrtc::AudioDecoderFactory>& decoder_factory,
+    rtc::scoped_refptr<webrtc::AudioMixer> audio_mixer,
     VoEWrapper* voe_wrapper)
     : adm_(adm), decoder_factory_(decoder_factory), voe_wrapper_(voe_wrapper) {
   RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());