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uss_migrator is responsible for migration (meta)data from the directory
to the USS architecture upon first time switching to USS.
USS assumes that all bookmarks have valid unique_position.
This CL adds a DCHECK that this is actually the case.
Directory is responsible for assigning valid unique positions for
legacy bookmarks that are missing the field.

Bug: 516866
Change-Id: Ib963016a56c4c9275649a1ff78f440d7fbe23023
Reviewed-on: https://chromium-review.googlesource.com/1240296
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593548}