Limit traversal statistics dumps to subtrees of 50 or more elements.

On Gecko, we get tons of console spam from 1-element traversals if we
don't do this.

components/style/parallel.rs

@@ -86,7 +86,9 @@ pub fn traverse_dom<E, D>(traversal: &D,
             }
         });
         aggregate.finish(traversal, start_time.unwrap());
-        println!("{}", aggregate);
+        if aggregate.is_large_traversal() {
+            println!("{}", aggregate);
+        }
     }
 }