Create a HandleScope when serializing an object's contents, to reduce
the number of live handles during serialization. There's only a couple
of cases where these handles have to outlive the serialized contents,
and for these cases we introduce GlobalHandleVector or similar manual
strong root mechanisms.

In particular, backrefs don't actually need to exist as a handle vector
(the object addresses are already referred to by the reference map's
IdentityMap), except for DCHECKs, so this becomes a DEBUG-only global
handle vector.

To support this manual strong-rooting, the HotObjectList is split up
into a strong-rooted find-only class in Serializer, and a Handle
vector in Deserializer.

Bug: chromium:1075999
Change-Id: I586eeeb543e3f6c934c168961b068f2c34e72456
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449980Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70411}