Allow Chrome to block navigations toward external protocols in sandboxed iframes

Chrome will block navigations toward external protocols inside sandboxed iframe. See https://chromestatus.com/features/5680742077038592. When True, this lets Chrome blocks those navigations. When False, this prevents Chrome from blocking those navigations. This defaults to True: security feature enabled. This can be used by administrators who need more time to update their internal website affected by this new restriction. This Enterprise policy is temporary; it's intended to be removed after Brave version 117.