Have CharacterData call children_changed on its parent when data is set.

Have CharacterData.SetData call children_changed on its parent when
data is set (if it is a Text node) so that HTMLStyleElement parents can
re-parse. Add variant ChildrenMutation::Text for it to use as the
mutation.

This fixes an issue where an empty <style> element's data is set but the
style is not updated. An HTMLStyleElement parent re-parses in its
children_changed implementation.

components/script/dom/nodelist.rs

@@ -279,6 +279,7 @@ impl ChildrenList {
                     self.last_index.set(middle as u32);
                 }
             },
+            ChildrenMutation::ChangeText => {},
         }
     }