AtomicCtorTag is needed by Blink to force atomic initialization of
members. This is used when reinitializing a member in a backing store.

Bug: chromium:1056170
Change-Id: I410766a9c9133a1f1c2ea2e1153cb1c61363459f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2859944
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74341}