BasicMemoryChunk will become the base class for ReadOnlySpace pages
which won't be MemoryChunks. Since ReadOnlySpace pages don't participate
in GC they don't need slot_set_.

Also some references to BasicMemoryChunk fields that were still prefixed
with MemoryChunk::

Bug: v8:10454
Change-Id: If8ce40c7ee72d1617d2a1161ad9d4b7929f8a8e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2198988
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67774}