Returned LABs can be of size less than sizeof(FreeListEntry).

Bug: chromium:1056170
Change-Id: Ib4094701472ce7cb5ee20b9fe632651570832dc9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2183051
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67635}