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Preference can be deleted during sync rollback if the preference was not set before
first signin. Clear the preference if that happens.

BUG=362679

Review URL: https://codereview.chromium.org/444763005

Cr-Commit-Position: refs/heads/master@{#289048}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289048 0039d316-1c4b-4281-b951-d872f2087c98