This doc tried to navigate the top page when loaded, which should fail since it's not trigged by user activation while in a sandboxed frame with 'allow-top-navigtaion-by-user-activation'.

Click the button below, the top navigation should succeed with a new page saying "PASSED: Navigation succeeded." in browsers supporting the 'allow-top-navigtaion-by-user-activation' iframe@sandbox keyword (eg., Chrome v58+); Otherwise, the top navigation should fail.