Note that `StyleRule` may not have the `CssRules` readily available,
they may need to be created. So the previous approach of providing
`CSSGroupingRule` with the `CssRules` is no good: it would require
writing them in advance, just in case they end up being used.
Therefore, this removes the `CSSGroupingRule::rules` field. Instead,
they are lazily obtained in `CSSGroupingRule::rulelist()` by downcasting
and calling the appropriate method for the subclass.
Testing: covered by WPT
Fixes: #36245
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
So that we can see the improvement when enabling the feature.
Testing: This PR enables tests
This is part of #36245
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
