counter_ could never be RO_SPACE. Make sure RO_SPACE and OLD_SPACE are
marked as unreachable.

Added tests for PagedSpaces and SpaceIterator.

Bug: v8:9183
Change-Id: I97bc2b4e0e5af37363a1c628ca7d69d2790a97b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1635696
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61943}