This patch gives DescriptorArray its own visitor id and its
own layout that is independent from the layout of WeakFixedArray.
This allows us to use raw 16-bit integers for keeping track of
the number of descriptors (total, non-slack, and marked).

As a side-effect, we save one word per descriptor array on 64-bit.

v8:8486

Change-Id: If8389dde446319e5b3491abc948b52539dba235c
Reviewed-on: https://chromium-review.googlesource.com/c/1349245
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57845}