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}