style: Expose the traversal kind to the style system.

This way we'll be able to take different paths for the sequential and parallel
traversals in some concrete cases.

This is a preliminar patch to fix bug 1332525.

components/style/sequential.rs

@@ -18,6 +18,7 @@ pub fn traverse_dom<E, D>(traversal: &D,
     where E: TElement,
           D: DomTraversal<E>,
 {
+    debug_assert!(!traversal.is_parallel());
     debug_assert!(token.should_traverse());
 
     fn doit<E, D>(traversal: &D, traversal_data: &mut PerLevelTraversalData,