Bug 1442404: Don't spin STS when shutting down the mtransport thread. r=drno

MozReview-Commit-ID: 47BImEsl9xi

--HG--
extra : rebase_source : 9fd820f4db145642f1184264ae991ea0748b0479

media/mtransport/nr_socket_prsock.cpp

@@ -233,7 +233,7 @@ public:
       // in-use -> idle -- no one forcing it to remain instantiated
       r_log(LOG_GENERIC,LOG_DEBUG,"Shutting down wrapped SingletonThread %p",
             mThread.get());
-      mThread->Shutdown();
+      mThread->AsyncShutdown();
       mThread = nullptr;
       // It'd be nice to use a timer instead...  But be careful of
       // xpcom-shutdown-threads in that case