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The old migration way requires changing of sync settings to complete
the migration (https://codereview.chromium.org/11961030/). That way
also blocks updates in SyncPrefs class required for crbug.com/906611
since it uses SyncPrefs internal logic. New way is implemented outside
of SyncPrefs and force the migration on browser start.

Bug: 906611
Change-Id: I0518798c5246e6436449ab79ddd648e68a52507c
Reviewed-on: https://chromium-review.googlesource.com/c/1434281Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#626112}