Splits heap setup to enable a single setup method on read-only heap.
This simplifies shared read-only heap initialization code.

Bug: v8:7464
Change-Id: If4f61e1fbc4780e19dcda2b2d50050b2c204b0e3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1518179
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60224}