Passed AudioMixer to AudioState::Config.

webrtc/audio/audio_state.cc

Index: webrtc/audio/audio_state.cc
diff --git a/webrtc/audio/audio_state.cc b/webrtc/audio/audio_state.cc
index 95a90a5bf1153ba0d51b1c669cdac0a8936d59a8..c15ddd7795013151bf51c095588ea3c16bc0d7dc 100644
--- a/webrtc/audio/audio_state.cc
+++ b/webrtc/audio/audio_state.cc
@@ -26,6 +26,8 @@ AudioState::AudioState(const AudioState::Config& config)
                              voe_base_->audio_processing(),
                              config_.audio_mixer) {
   process_thread_checker_.DetachFromThread();
+  RTC_DCHECK(config_.audio_mixer);
+
   // Only one AudioState should be created per VoiceEngine.
   RTC_CHECK(voe_base_->RegisterVoiceEngineObserver(*this) != -1);
 
@@ -48,6 +50,7 @@ VoiceEngine* AudioState::voice_engine() {
 }
 
 rtc::scoped_refptr<AudioMixer> AudioState::mixer() {
+  RTC_DCHECK(thread_checker_.CalledOnValidThread());
   return config_.audio_mixer;
 }