Initialize thread-safe count properly.

Bug: v8:12620
Change-Id: Ifb43a860f1b8cefd410fea25ac408f5be55ab1af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3455823
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79046}