Once read-only space is not a Heap space it makes little sense to have
it in the Heap class.

Bug: v8:7464
Change-Id: I2230ce7cbf1cec3c83065c91bc14a9c23f72478b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1688841Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#62540}