Fix calculation of base styles to drop animation rules

It seems that changeset 97ce9ed
mistakenly changed the check that a cascade level to keep is *not* an
animation level to a check that it *is* an animation level.

components/style/rule_tree/mod.rs

@@ -369,7 +369,7 @@ impl RuleTree {
         let mut last = path;
         let mut children = vec![];
         for node in iter {
-            if node.cascade_level().is_animation() {
+            if !node.cascade_level().is_animation() {
                 children.push((node.get().source.clone().unwrap(), node.cascade_level()));
             }
             last = node;