style: Restyle pseudo-elements as well on part attribute changes

Refactor a bit the code to unify how we deal with this conditional
restyling (we had similar code for
MustCascadeChildrenIfInheritResetStyle).

Differential Revision: https://phabricator.services.mozilla.com/D172890

components/style/matching.rs

@@ -75,13 +75,6 @@ pub enum ChildRestyleRequirement {
     MustMatchDescendants = 4,
 }
 
-impl ChildRestyleRequirement {
-    /// Whether we can unconditionally skip the cascade.
-    pub fn can_skip_cascade(&self) -> bool {
-        matches!(*self, ChildRestyleRequirement::CanSkipCascade)
-    }
-}
-
 /// Determines which styles are being cascaded currently.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 enum CascadeVisitedMode {