Do the sequential traversal breadth-first.

While we're at it, we also eliminate the 'unknown' dom depth for the
bloom filter. Computing depth has negligible cost relative to the
amount of work we do setting up the bloom filter at a given depth.
Doing it once per traversal should be totally fine.

I originally separated the elimination of unknown dom depth from the
traversal changes, but I got bloom filter crashes on the intermediate
patch, presumably because I didn't properly fix the sequential traversal
for this case. Given that the final state is green, I just decided to
squash and move on.

components/layout/traversal.rs

@@ -56,7 +56,7 @@ impl<'a, E> DomTraversal<E> for RecalcStyleAndConstructFlows<'a>
 {
     type ThreadLocalContext = ScopedThreadLocalLayoutContext<E>;
 
-    fn process_preorder(&self, traversal_data: &mut PerLevelTraversalData,
+    fn process_preorder(&self, traversal_data: &PerLevelTraversalData,
                         thread_local: &mut Self::ThreadLocalContext, node: E::ConcreteNode) {
         // FIXME(pcwalton): Stop allocating here. Ideally this should just be
         // done by the HTML parser.