Documents created via DOMParser.parseFromString()
are parsed synchronously and do not participate in the
browsing context's loading pipeline.
This patch ensures that if the document has no browsing context
(i.e. was parsed via DOMParser),
its readiness is set to "complete" synchronously.
Fixes WPT:
domparsing/xmldomparser.html
This is required by mini Cloudflare invisible challenges, as it will
only run if the readyState is not "loading". If it is "loading", then
it waits for readystatechange to check that it's not "loading" anymore.
Initial about:blank iframes do not go through the full navigation and
thus don't go through HTMLParser::the_end, which sets the ready state
to something other than "loading". Therefore, the challenge would never
run, as readyState would never change.
Seen on https://discord.com/login
