This patch also makes RulesIterator not iterate over rules for which we don't
process nested rules.
There's nothing depending on this behavior right now afaik, and this will make
us duplicate less code in following patches.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1357461
MozReview-Commit-ID: CaMFQtAVnJF
This is needed for both bug 1357461 and bug 1273303, where I plan to add smarter
invalidations than what we have now.
Also, it's cleaner.
Ideally I'll move this onto stylist, though that may require extra work to make
it work fine for Servo, so for now let's just do the obvious thing.
