As it stands most of ReadOnlySpace class's method are unusable once it
has been sealed, since all of its pages are read-only. Set owner_ to null
to ensure nothing unintentionally uses it.

This also helps with separating the ReadOnlySpace from the Heap class in
the future as ReadOnlySpace might not inherit from Space.

Bug: v8:7464
Change-Id: I3b24f20c644d6f5e23647bc1de4d256a20a0eb19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1637800Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#61979}