Use nsIClassOfService flags to check if a request is urgent, or can be batched:
Leader | Unblocked | UrgentStart | TailForbidden
These cover top frame html, <head> blocking css and js (and xhr), font preloads,
and everything else that can delay first paint or DOMContentLoaded.
Differential Revision: https://phabricator.services.mozilla.com/D111744