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Since |u2f_request_| is assigned to the result of |TryRegistration| /
|TrySign|, if the given callback is called immediately then |Cleanup|
can have run and completed before |u2f_request_| is assigned. That
leaves |AuthenticatorImpl| in an invalid state and it'll no longer
process any requests.

None of the existing callbacks do this, so this change just adds DCHECKs
to help anyone else who might hit this in the future.

Change-Id: I139a0825c6231c438dc9e469f9b188eddb6000e0
Reviewed-on: https://chromium-review.googlesource.com/961788
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543036}