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/1264698Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56494}