Now that BeginUpdate is useless for the UPDATE_STYLE case, we don't need the
update mechanism at all. Just ensure that ApplicableStylesChanged is called on
the pres shell via the relevant RuleChanged, etc. notifications.

There's a big hidden gotcha here. nsIDocument::BeginUpdate does put a script
blocker on the stack for these updates. However it's not needed, since no script
can run during these notifications (only the stylesheet events we post for
devtools, but those use AsyncEventDispatcher and PostDOMEvents, so they don't
try to run immediately).

nsIDocument::BeginUpdate also does XBL binding attached queue stuff, but we
can't change bindings during these notifications anyway, so it also doesn't
matter.

MozReview-Commit-ID: HJvK6zQfloh