Read-only space sharing requires an iterator independent of heap. This
also enables future removal of read-only space from heap.

Bug: v8:7464
Change-Id: Ia07a9369494ea2c547d12c01ffa1d7b8b6bbeabc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1552795
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60819}