Instead of skipping LAB in PagedSpaceObjectIterator, make the space
iterable by inserting a filler object into the LAB.

Bug: v8:10315
Change-Id: I6d79c309b7b8180b2a173ebd5ebdf8a893e88c4d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2210234Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67928}