Reduce the granularity to 4 bytes:
- Saves some memory on 32bit configurations
- Allows uniformly reasoning about HoH::ObjectSize() with
  RoundUp<kAllocationGranularity>(sizeof(T))

Change-Id: Ic87aa25839d9b8a99916c07d64e5e49864a6cb53
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2739628Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73267}