This reverts commit 40f1aaf3.

Reason for revert:
https://luci-milo.appspot.com/buildbot/client.v8/V8%20Linux%20-%20arm64%20-%20sim%20-%20MSAN/21000

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}

TBR=yangguo@chromium.org,hpayer@chromium.org,mlippautz@chromium.org,delphick@chromium.org

Change-Id: Id4770c0fdb21cd9eea2f62a019f44a6bdea8f0a7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7464
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1041948Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52944}