A SafepointScope might need to block for a shared GC initiated from
another client isolate. This means that anytime we create a
SafepointScope a shared GC may run. This CL adds a DCHECK to ensure
AllowGarbageCollected::IsAllowed() holds for each SafepointScope.

So far this DCHECK was only run in the less likely event that a
SafepointScope actually runs a shared GC. Which is technically good
enough but it is easy to miss use cases of SafepointScope where this
does not hold.

Bug: v8:11708, v8:12377
Change-Id: I30cc33c05ebe4835430e1d699a86079810523858
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3289625Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77976}