When deserializing the startup snapshot, call IterateStrongRoots with
VISIT_FOR_SERIALIZATION rather than VISIT_ONLY_STRONG. To compensate,
make the StartupDeserializer explicitly iterate over the partial
snapshot cache.

This makes the deserializer and serializer consistent in their use of
the function and makes their differences explicit in the snapshot code
itself.

Bug: chromium:902230
Change-Id: I3a2ac858f4f6b3097b98a10ed2dd5ac5b9bf83e8
Reviewed-on: https://chromium-review.googlesource.com/c/1319585
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57275}