stylo: Cleanup a bit of the Stylist clear setup.

This moves us to clear on rebuild, which allows us to remove yet another place
where we track stylist dirtiness.

Bug: 1390255
Reviewed-by: heycam
MozReview-Commit-ID: nihQbUAbh8
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/selector_map.rs

@@ -123,6 +123,15 @@ impl<T> SelectorMap<T> {
         }
     }
 
+    /// Clears the hashmap retaining storage.
+    pub fn clear(&mut self) {
+        self.id_hash.clear();
+        self.class_hash.clear();
+        self.local_name_hash.clear();
+        self.other.clear();
+        self.count = 0;
+    }
+
     /// Returns whether there are any entries in the map.
     pub fn is_empty(&self) -> bool {
         self.count == 0