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This adds a State enum to StartupController and exposes it via GetState().
It's not strictly required right now, but
a) it'll help with sanitizing ProfileSyncService's state,
b) it makes the tests a bit nicer, now they can use the enum instead of
   relying on debug strings, and
c) it lets us move GetEngineInitializationStateString from
   StartupController to ProfileSyncService, where it's slightly less
   out of place (bonus: extra DCHECKs for consistent state).

Other semi-related test cleanups: don't call SetFirstSetupComplete from
FakeStartBackend (no idea why that call was there), and don't check for
the kSyncDisableDeferredStartup cmdline flag (if you run the tests with
a custom flag, it's your own fault if they fail).

Bug: 839834
Change-Id: I16f83be8ee828024329cbce899be4723241323db
Reviewed-on: https://chromium-review.googlesource.com/1109939
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569273}