Bug 1854757 - Relax assert to account for snapshots that we take for relative selector invalidation. r=dshin

Not sure how easy would it be to construct a test-case for this...

Differential Revision: https://phabricator.services.mozilla.com/D189061

servo/components/style/traversal.rs

@@ -405,6 +405,7 @@ pub fn recalc_style_at<E, D, F>(
     context.thread_local.statistics.elements_traversed += 1;
     debug_assert!(
         flags.intersects(TraversalFlags::AnimationOnly) ||
+            is_initial_style ||
             !element.has_snapshot() ||
             element.handled_snapshot(),
         "Should've handled snapshots here already"