Opt-in rather than opt-out to unsafe blocks in layout.

components/layout/util.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![allow(unsafe_blocks)]
+
 use construct::ConstructionResult;
 use incremental::RestyleDamage;
 use parallel::DomParallelInfo;