style: Make stylesheet invalidation faster.

This fixes performance issues when there's a massive number of rules and
a massive DOM.

Instead of going through all rules for all the DOM, we now look stuff in
hashmaps.

Differential Revision: https://phabricator.services.mozilla.com/D83506

components/style/selector_map.rs

@@ -620,13 +620,20 @@ pub struct MaybeCaseInsensitiveHashMap<K: PrecomputedHash + Hash + Eq, V: 'stati
     PrecomputedHashMap<K, V>,
 );
 
+impl<V: 'static> Default for MaybeCaseInsensitiveHashMap<Atom, V> {
+    #[inline]
+    fn default() -> Self {
+        MaybeCaseInsensitiveHashMap(PrecomputedHashMap::default())
+    }
+}
+
 // FIXME(Manishearth) the 'static bound can be removed when
 // our HashMap fork (hashglobe) is able to use NonZero,
 // or when stdlib gets fallible collections
 impl<V: 'static> MaybeCaseInsensitiveHashMap<Atom, V> {
     /// Empty map
     pub fn new() -> Self {
-        MaybeCaseInsensitiveHashMap(PrecomputedHashMap::default())
+        Self::default()
     }
 
     /// HashMap::try_entry
@@ -641,6 +648,12 @@ impl<V: 'static> MaybeCaseInsensitiveHashMap<Atom, V> {
         self.0.try_entry(key)
     }
 
+    /// HashMap::is_empty
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     /// HashMap::iter
     pub fn iter(&self) -> hash_map::Iter<Atom, V> {
         self.0.iter()