Add a mutex lock to AudioPowerMonitor::Reset() to placate TSAN.

media/audio/audio_power_monitor.cc

Index: media/audio/audio_power_monitor.cc
diff --git a/media/audio/audio_power_monitor.cc b/media/audio/audio_power_monitor.cc
index 6536f464b9c16905e7d4080a907b5a4df5678acf..8b7cbffd975c479887549575ab649e072a9b6c76 100644
--- a/media/audio/audio_power_monitor.cc
+++ b/media/audio/audio_power_monitor.cc
@@ -26,6 +26,7 @@ AudioPowerMonitor::~AudioPowerMonitor() {
 }
 
 void AudioPowerMonitor::Reset() {
+  base::AutoLock for_reset(reading_lock_);
   power_reading_ = average_power_ = 0.0f;
   clipped_reading_ = has_clipped_ = false;
 }