This reverts commit 964edc25.

Reason for revert: chromium:959190

Original change's description:
> [heap] Set read-only space's and its pages' heap_ to null.
>
> 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}

TBR=ulan@chromium.org,hpayer@chromium.org,delphick@chromium.org,goszczycki@google.com

Change-Id: I53cecf3976dfeabae309040313351385f651f010
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7464, chromium:959190
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1591608Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61217}