Bug 1579858 - Should release RefPtr before CollectData is clear; r=smaug

mAboutToBeNotifiedRejectedPromises will be clear in AfterProcessMicrotasks()
and mPendingUnhandledRejections will be clear after NotifyUnhandledRejections
runnable is handled.

However, worker could terminate in any time, we still need to clear those
structures manually before CollectData is clear.

Differential Revision: https://phabricator.services.mozilla.com/D46095

--HG--
extra : moz-landing-system : lando