Rename PrivateStyleData to PersistentStyleData and use AtomicRefCell instead of RefCell as a container.

This allows us to eliminate the unsafe borrows. \o/

MozReview-Commit-ID: 29hpGaWUFQz

components/style/parallel.rs

@@ -161,9 +161,7 @@ fn bottom_up_dom<N, C>(root: OpaqueNode,
             Some(parent) => parent,
         };
 
-        let parent_data = unsafe {
-            &*parent.borrow_data_unchecked().unwrap()
-        };
+        let parent_data = parent.borrow_data().unwrap();
 
         if parent_data
             .parallel