style: Shrink maps if needed after stylist rebuilds

Hashbrown grows a lot sometimes making us waste a lot of memory. Shrink
some of these maps after CascadeData rebuild / stylesheet collection
invalidation.

Differential Revision: https://phabricator.services.mozilla.com/D134716

components/style/selector_parser.rs

@@ -170,9 +170,14 @@ impl<T> PerPseudoElementMap<T> {
     }
 
     /// Get an iterator for the entries.
-    pub fn iter(&self) -> ::std::slice::Iter<Option<T>> {
+    pub fn iter(&self) -> std::slice::Iter<Option<T>> {
         self.entries.iter()
     }
+
+    /// Get a mutable iterator for the entries.
+    pub fn iter_mut(&mut self) -> std::slice::IterMut<Option<T>> {
+        self.entries.iter_mut()
+    }
 }
 
 /// Values for the :dir() pseudo class