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 4bd96cd72f5a72657a80c0f2639f726dec294758..f6bb23294682748215505cc28240f3ac7eb0d09a 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_impl.cc
+++ b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
@@ -380,4 +380,13 @@ 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