This fixes one source of flakiness on WPT (of many) where we wouldn't
recompute style after programmatically altering the contents of a style
sheet, but instead had to wait for something else to cause invalidation.
The regression in the "conditional-CSSGroupingRule" test is we now fail
the "inserting an `@import`" subtests differently and the subtests
aren't independent. Specifically, we don't yet implement the checks in
`CSSRuleList::insert_a_css_rule()` that reject certain rules from being
inserted. Previously we didn't insert the `@import` rule because we
failed to parse its relative URL. Now we parse it correctly, we end up
inserting it.
