Now ReadOnlySpace is not a Space, it cannot be upcasted to it, so cast
to BaseSpace as well.

Fixes ubsan failure introduced by
https://chromium-review.googlesource.com/c/v8/v8/+/2209060.

Bug: v8:10454
Change-Id: I2936bfd8349377b441be4a6eedf580d91f1c4ee4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228723
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68140}