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Since the current implementation doesn't support non-U2F authenticators,
all non-U2F scenarios can be immediately denied with NotSupportedError.

This CL also moves handling for empty allowCredentials (part of the
aforementioned errors) out of u2f_sign, since u2f_sign should never be
called with empty allowCredentials.

Bug: 819256
Change-Id: I991949919fbe7a5e1e9c123ee750d80c3d25e9c3
Reviewed-on: https://chromium-review.googlesource.com/1006482
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550346}