This only changes the behaviour when called with a TaskQueue or other type
using SerialEventTargetGuard on the stack. They are being switched over as the
existing GetCurrentEventTarget method is being removed, as it is somewhat
confusing, and poorly documented.
Callers which need to get the current thread even when on a threadpool or
behind a TaskQueue were switched to GetCurrentEventTarget in the previous part.
Differential Revision: https://phabricator.services.mozilla.com/D166607
