and
1) make them represent kSystemPointerSize-sized off-heap slots,
2) reintroduce XxxSlots as an on-heap kTaggedSize-sized slots (for now they
   are just aliases to respective FullXxxSlots).

Bug: v8:8518
Change-Id: I8a9177562308bd9420b1eebca959cc52ceaa628e
Reviewed-on: https://chromium-review.googlesource.com/c/1363144
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58056}