message: Do not allow sending tasks to a thread that is quitting.
  Cr-Commit-Position: refs/heads/master@{#30302}
diff: @@ -844,6 +844,7 @@ void Thread::Send(const Location& posted_from,
  MessageHandler* phandler,
  uint32_t id,
  MessageData* pdata) {
  + RTC_DCHECK(!IsQuitting());
  if (IsQuitting())
  return;
  