... to avoid it being accessible from BG threads.

Bug: v8:7700
Change-Id: I8fa7ac66016cb1f70925ed84a6552128d419f330
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899128Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83356}