This moves several fields that will be needed by ReadOnlySpace pages
when it stops using MemoryChunk into BasicMemoryChunk.

Additionally AllocationStats is moved from spaces.h into
allocation-stats.h.

Bug: v8:10473, v8:10454
Change-Id: I76a66565a260126e629bd7588a5418267dfa8423
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228722Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68134}