Sorry this is not a particularly easy patch to review. But it should be
mostly straight-forward.
I kept Document::Dispatch mostly for convenience, but could be
cleaned-up too / changed by SchedulerGroup::Dispatch. Similarly maybe
that can just be NS_DispatchToMainThread if we add an NS_IsMainThread
check there or something (to preserve shutdown semantics).
Differential Revision: https://phabricator.services.mozilla.com/D190450
