A shared heap will not have a young generation in the beginning.

Bug: v8:11708
Change-Id: I947ddb91a23a72a8cee3aa3e554723dda8146011
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891569
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74697}