Weak containers are re-traced during conservative stack scanning to
strongify their contents. This runs in parallel with concurrent
marking that could find new weak containers.

The CL fixes two issues:
- The concurrent marker could find a weak container and mark it but
  would only add it to the set of weak containers afterwards. We need to
  reverse this (using a lock) to make sure that the main thread sees the
  marked weak container.
- The DCHECK for containment needs to be concurrency aware.

Bug: chromium:1349298
Change-Id: I1ec31db62647f7f1c19e9cc60976e09946551333
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3807593Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82170}