There are some functions in packet_router.cc and modules/congestion_controller that could be used by different threads, but they're protected using rtc::ThreadChecker which doesn't allow them to be called by more than one thread even if the calls are synchronised. This CL replaces those with rtc::RaceChecker, which allows serialized access of the functions from multiple threads.
BUG=webrtc:7826
Review-Url: https://codereview.webrtc.org/2940133003
Cr-Commit-Position: refs/heads/master@{#18628}
bf5a2fc11b