Up until now the first page of ReadOnlySpace was not read-only.
Changing this means that the space needs to be made writable before
PagedSpace's TearDown tries to free it.

Bug: v8:7464
Change-Id: I162b45b662a6e5ab61a35be280ef087375fdcc9b
Reviewed-on: https://chromium-review.googlesource.com/c/1454494Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#59432}