Assert that compute_style_difference is only called with eager pseudos.

Gecko bug 1324618 part 3: https://bugzilla.mozilla.org/show_bug.cgi?id=1324618

components/style/matching.rs

@@ -1478,6 +1478,7 @@ pub trait MatchMethods : TElement {
                                 pseudo: Option<&PseudoElement>)
                                 -> StyleDifference
     {
+        debug_assert!(pseudo.map_or(true, |p| p.is_eager()));
         if let Some(source) = self.existing_style_for_restyle_damage(old_values, pseudo) {
             return RestyleDamage::compute_style_difference(source, new_values)
         }