In a previous CL (crrev.com/c/v8/v8/+/2424130/17/src/objects/map.h) I
moved it to public to use the parent macro. Move it back to private
since it does not need to be public.

Bug: v8:7790
Change-Id: I5fd78a8fcef2bd454585b6dc428c1b1d2bb62422
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2454715
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70377}