After https://chromium-review.googlesource.com/c/v8/v8/+/2250254,
allocated_object_size in RO_SPACE is incorrect. This changes it to use
the accounting_stats_ value. This also fixes the Capacity() which was
previously uninitialized. Both are tested in new ReadOnlySpace allocation
tests in test-spaces.cc.

Couple of cleanups:
* area_size_ becomes const since its value is fixed after construction.
* Deletes incorrect comment in base-space.h

Bug: v8:10454
Change-Id: I9bbbc1ef2548722eee9dae1bb8d67448eccf8955
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2259937
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68489}