Poisoning unmarked objects serves two purposes:
- Prohibits finalizer from accessing other unmarked objects;
- Unpoisioning also clears potential poisoning of the embedder which
  is necessary as the sweeper and compactor modify the payload of
  objects;

Bug: chromium:1056170
Change-Id: I4346a0ab736603b3d6170b41b0e7255db1452897
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2762137Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73426}