The generational barrier isn't supported on the background thread at the
moment. Make sure it isn't used on such threads by accident.

Bug: v8:13203
Change-Id: I5577f3802c1aba246955519c8c778fa741d56d96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3840300
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82635}