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This changes the return values of
FidoAuthenticator::AuthenticatorTransport() and
AuthenticatorMakeCredentialResponse::transport_used() to
base::Optional<FidoTransportProtocol>.

This accommodates the Windows API integration use case where (a) the
FidoAuthenticator subclass cannot guarantee any particular transport
type and (b) the response we receive from the API contains no hints
about which transports were used.

Bug: 898718
Change-Id: Iadb83310f03d5f54ad50babb73e0963bdd21e3c2
Reviewed-on: https://chromium-review.googlesource.com/c/1325220Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607463}