Field used to be atomic for AssertActive() but now that this method
uses another approach, we can make that field non-atomic again.

Bug: v8:11708
Change-Id: I5254aa5e655844739082144fc75ba9f0c13d1ba0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3317424Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78273}