We must ensure required root objects will be filled when such
objects will be deserialized.

Change-Id: I25136d31cb2e0c0a69a51c5635192f17bbe2a9ba
Reviewed-on: https://chromium-review.googlesource.com/579768
Commit-Queue: Vyacheslav Chigrin <vchigrin@yandex-team.ru>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46944}