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This implements the WebauthN spec change that permits returning an immediate
and specific error if the user consented to use a key but it was already
registered with the relying party. Formerly, we had to wait for a timeout
and return the generic "NotAllowedError". This way, RPs can let the user
know the reason that registration failed.

Bug: 814870
Change-Id: Ib6f0c9cdd5ca7e3f545c8ce6f3c8e641d672783b
Reviewed-on: https://chromium-review.googlesource.com/984725Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546976}