Until now we asserted that EnsureMTA was being called from the main thread
unless the current thread was already in the MTA.
This patch makes MTA thread initialization thread-safe by using a C++11
"magic static," thus allowing EnsureMTA to be safely invoked from any thread.
Differential Revision: https://phabricator.services.mozilla.com/D38422
--HG--
extra : moz-landing-system : lando
