Adding some DCHECKs to document invariants and correctness properties
(e.g. pages with live objects aren't freed).

Bug: v8:12612
Change-Id: I543e4846c791320f3965561ae9d0b54739f5df03
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3507993Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79542}