Make sure there is no background GC when setting flags.

Bug: v8:12612, v8:13185
Change-Id: I0a2d4796abe265defa00d86f826003eb048e5bf1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829482
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82474}