Avoid race with concurrent allocation thread by updating the flag
conditionally.

Bug: v8:10315
Change-Id: I63df8ab5a6a506e9337281c9fef7555b9deaaee6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424264Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70058}