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This is a follow up to https://chromium-review.googlesource.com/1238499.

The update of the sync settings during migration can happen at the
same time as updating the sync settings because of a sync state change.

To resolve inconsistencies in the configuration updates, the update
method UpdateSyncSettingsIfPossible now takes different parameters.

Bug: 886912, 887706
Change-Id: I27555172164b99332cdce101f79564e58cc05eff
Reviewed-on: https://chromium-review.googlesource.com/1238572
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593518}