Replace begin_styling with a centralized mechanism to set a node up for either styling or restyling.

We also make sure that an element never has an ElementData with ElementDataStyles::Uninitialized,
and eagerly call prepare_for_styling whenever an ElementData is instantiated.

MozReview-Commit-ID: 9YP6eSmdMt0

components/style/gecko/traversal.rs

@@ -55,7 +55,7 @@ impl<'lc, 'ln> DomTraversalContext<GeckoNode<'ln>> for RecalcStyleOnly<'lc> {
         }
     }
 
-    fn ensure_element_data<'a>(element: &'a GeckoElement<'ln>) -> &'a AtomicRefCell<ElementData> {
+    unsafe fn ensure_element_data<'a>(element: &'a GeckoElement<'ln>) -> &'a AtomicRefCell<ElementData> {
         element.ensure_data()
     }