Migrate pc/ to webrtc::Mutex.

Bug: webrtc:11567
Change-Id: I1adc22d2998966958750138e66108cf39a8c3d57
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178840
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31654}
diff --git a/pc/remote_audio_source.h b/pc/remote_audio_source.h
index 15dc75b..9ec0916 100644
--- a/pc/remote_audio_source.h
+++ b/pc/remote_audio_source.h
@@ -18,8 +18,8 @@
 #include "api/call/audio_sink.h"
 #include "api/notifier.h"
 #include "pc/channel.h"
-#include "rtc_base/critical_section.h"
 #include "rtc_base/message_handler.h"
+#include "rtc_base/synchronization/mutex.h"
 
 namespace rtc {
 struct Message;
@@ -69,7 +69,7 @@
   rtc::Thread* const main_thread_;
   rtc::Thread* const worker_thread_;
   std::list<AudioObserver*> audio_observers_;
-  rtc::CriticalSection sink_lock_;
+  Mutex sink_lock_;
   std::list<AudioTrackSinkInterface*> sinks_;
   SourceState state_;
 };