This type is a lot of complexity related to a very specific thing such as the
hover and active quirk.
Instead of that, move `nesting_level` to `MatchingContext`, and simplify all
this computing whether the quirk applies upfront, for each complex selector we
test.
This is less error-prone, and also allows simplifying more stuff in a bit.
Otherwise, tests like the following fail, given we always match as unvisited,
and we'd never mark the link as needing invalidation.
<!doctype html>
<style>
a {
color: red !important;
}
.foo :visited {
color: green !important;
}
</style>
<div>
<a href="https://google.es">visit me</a>
<button onclick="this.parentNode.className = 'foo'">Then click me</button>
</div>
Bug: 1368240
MozReview-Commit-ID: LDv6S28c4ju
