Do not allow sending tasks to a thread that is quitting.

Bug: webrtc:9883
Change-Id: I17c7a447cf6982148e20a53a3a3aaaeebe8bf233
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165782
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30302}

rtc_base/thread.cc

@@ -844,6 +844,7 @@ void Thread::Send(const Location& posted_from,
                   MessageHandler* phandler,
                   uint32_t id,
                   MessageData* pdata) {
+  RTC_DCHECK(!IsQuitting());
   if (IsQuitting())
     return;