Replace AudioConferenceMixer with AudioMixer.

webrtc/modules/audio_mixer/audio_mixer_impl.cc

Index: webrtc/modules/audio_mixer/audio_mixer_impl.cc
diff --git a/webrtc/modules/audio_mixer/audio_mixer_impl.cc b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
index d7025f9e671279fc73bd7152f55663228c8a005a..7922feaaa653b2626ac8510dad100dd27c5cb8c6 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_impl.cc
+++ b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
@@ -381,4 +381,13 @@ bool AudioMixerImpl::GetAudioSourceMixabilityStatusForTest(
   LOG(LS_ERROR) << "Audio source unknown";
   return false;
 }
+
+bool AudioMixerImpl::GetSourcePresenceStatusForTest(
+    AudioMixerImpl::Source* audio_source) const {
+  RTC_DCHECK_RUNS_SERIALIZED(&race_checker_);
+  rtc::CritScope lock(&crit_);
+
+  const auto iter = FindSourceInList(audio_source, &audio_source_list_);
+  return iter != audio_source_list_.end();
+}
 }  // namespace webrtc