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This makes a few things a bit nicer: Now StartupController doesn't have
to know about the set of registered data types anymore, which are really
not its business. As a consequence, it's not necessary to call Reset()
before using the StartupController.
This also removes one of two uses of SyncPrefs from StartupController;
I hope to remove the remaining one soon.

Bug: 854978
Change-Id: I75c8971342554eb190287724eff2ff3238ffbff6
Reviewed-on: https://chromium-review.googlesource.com/1110220
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569337}