Various small changes are required to enable this.

HeapObject::GetReadOnlyRoots no longer uses the Space's heap when
possible (see comment in ReadOnlyHeap::GetReadOnlyRoots definition).
This requires that ReadOnlyRoots be construct-able using a raw pointer
to the read-only space's roots array.

Global read-only heap state is now cleared by tests where appropriate
and extra DCHECKs in ReadOnlyHeap::SetUp should make catching future
issues easier.

String padding is now always cleared just before read-only space is
sealed when not deserializing.

Change-Id: I7d1db1c11567be5df06ff7066f3a699125f8b372
Bug: v8:7464
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1535830
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@{#61188}