Now that we don't iterate over the strong roots in the StartupSerializer
twice, remove code related to skipping non-immortal immovable roots.
Factor out code from Serializer::VisitRootPointers and use that method in
the StartSerializer override.

Also update comments that reflected the old way of serializing.

Change-Id: Ieb5e63389f455b963244717cada7e5ccde8e41cb
Reviewed-on: https://chromium-review.googlesource.com/1179669
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55218}