This is a reland of 22b56f47

Nothing has changed. This is a reland of a speculative revert.

Original change's description:
> [snapshot] Factor out root serialization code
>
> Factors out a new method Serializer::SerializeRoot which attempts to
> serialize a given object as a Root if it is one and the Serializer's
> policy allows that root to be serialized (implemented as a new virtual
> method RootCanBeSerialized)..
>
> This is in preparation for adding a ReadOnlySerializer which change the
> way read-only roots are serialized.
>
> Bug: v8:8191
> Change-Id: I7fbb4e9520fba8b836a0b6bf95ca39abc3ded79e
> Reviewed-on: https://chromium-review.googlesource.com/c/1264698
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56494}

TBR=jgruber

Bug: v8:8191
Change-Id: I30a606b9c99f5651fae323d12f8f74b13bb085fa
Reviewed-on: https://chromium-review.googlesource.com/c/1273103Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56519}