PagedNewSpace requires a different way of iterating over new space.
This is because we can no longer assume that everything before top is
allocated and everything after is free.

Bug: v8:12612
Change-Id: Iedd36a6d3dc5019553f58f1ba9f5d06529a7ce9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3823129
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82357}