Add several thread checks to RtpSender classes.

Minor related updates to AudioTrack and VideoTrack's sequence checkers.

There's more that can be done (or arguably needs to), but this is
a start.

Bug: none
Change-Id: I3ccf8eb9bbb6bef62b83248a23a68871b9fcd9e1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251843
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36021}

pc/audio_track.h

@@ -17,6 +17,7 @@
 #include "api/media_stream_track.h"
 #include "api/scoped_refptr.h"
 #include "api/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -53,7 +54,7 @@ class AudioTrack : public MediaStreamTrack<AudioTrackInterface>,
 
  private:
   const rtc::scoped_refptr<AudioSourceInterface> audio_source_;
-  SequenceChecker thread_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker signaling_thread_checker_;
 };
 
 }  // namespace webrtc