Migrate content area accounts to crOS Account Manager

Add a |AccountMigrationRunner::Step| for migrating content area accounts
to Chrome OS Account Manager.

Please see the attached bug for a Design Document.

Bug: 904128
Change-Id: Ibda93ef5cd1dfbcd5785f4a4b832b9225bdcd7fe
Reviewed-on: https://chromium-review.googlesource.com/c/1375739
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621684}
1 file changed