style: Invalidate shadow part pseudo-class styles correctly.

I was going to send a test for `:focus` via wpt, but then realized it was
probably not spec-compliant with the new rules people want to follow for
:focus, so I filed https://github.com/w3c/csswg-drafts/issues/4555 instead.

Testing `:hover` / `:active` via wpt looked quite a bit of a hassle.

Differential Revision: https://phabricator.services.mozilla.com/D55591

components/style/stylist.rs

@@ -1889,18 +1889,33 @@ impl CascadeData {
         self.host_rules.as_ref().and_then(|d| d.rules(pseudo))
     }
 
+    /// Whether there's any host rule that could match in this scope.
+    pub fn any_host_rules(&self) -> bool {
+        self.host_rules.is_some()
+    }
+
     /// Returns the slotted rule map for a given pseudo-element.
     #[inline]
     pub fn slotted_rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
         self.slotted_rules.as_ref().and_then(|d| d.rules(pseudo))
     }
 
+    /// Whether there's any ::slotted rule that could match in this scope.
+    pub fn any_slotted_rule(&self) -> bool {
+        self.slotted_rules.is_some()
+    }
+
     /// Returns the parts rule map for a given pseudo-element.
     #[inline]
     pub fn part_rules(&self, pseudo: Option<&PseudoElement>) -> Option<&PartMap> {
         self.part_rules.as_ref().and_then(|d| d.rules(pseudo))
     }
 
+    /// Whether there's any ::part rule that could match in this scope.
+    pub fn any_part_rule(&self) -> bool {
+        self.part_rules.is_some()
+    }
+
     /// Collects all the applicable media query results into `results`.
     ///
     /// This duplicates part of the logic in `add_stylesheet`, which is