The generational write-barrier currently does not support background threads. As a result, building in the background a ConsString that
points to a young string can lead to bugs, since the young string could
be freed.

Bug: v8:13203
Change-Id: I0df7c8cca8712d765eff0b1e918379f5477fdee5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3840940Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82612}