This provides a single point where read-only space sharing will be
controlled. Eventually ReadOnlyDeserializer will take ReadOnlyHeap
instead of Isolate, first steps include
https://chromium-review.googlesource.com/c/v8/v8/+/1483054

Bug: v8:7464
Change-Id: I213819aeca6fca335235025c9195edf474230eda
Reviewed-on: https://chromium-review.googlesource.com/c/1489087
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59954}