This is a reland of 40f1aaf3

Put back padding clearing into the SerializeObject method but only when
the String is not in RO_SPACE. For RO_SPACE strings, if required
iterate over the space before serialization clearing the strings.

Original change's description:
> [heap] Mark RO_SPACE as read-only after deserialization
>
> Adds MarkAsReadOnly and MarkAsReadWrite to ReadOnlySpace. The latter
> is only usable with ReadOnlySpace::WritableScope to avoid the space
> being left writable). MarkAsReadOnly updates the high water mark and
> makes several previously mutating methods into no-ops.
>
> Moves some writes to immutable objects out of the bootstrapper to
> setup-heap-internal so they don't write to a read-only page.
>
> Also avoid writing hashes to strings that already have the value set as
> that invariably means writing to the "0" and "1" constant strings in
> RO_SPACE.
>
> Before serialization, it makes RO_SPACE writable again so that any
> padding can be cleared before writing it.
>
> Bug: v8:7464
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
> Change-Id: I22edc20dba7dde8943991a8fcaf87244af4490a3
> Reviewed-on: https://chromium-review.googlesource.com/1014128
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52943}

Bug: v8:7464
Change-Id: Ia8386c4ff5f5df3207f584caf7a9b1ff1e405f25
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1042145Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53087}