fido/mac: require user interaction before returning any error on GetAssertion

The TouchIdAuthenticator previously skipped the Touch ID prompt if no
matching credential existed for a GetAssertion request. This information
is now passed up into the UI at authenticator instantiation time, so the
authenticator does not need to handle it. Instead, it should implement
safe, spec-compliant behavior and *always* prompt the user before
returning an error.

Bug: 678128
Change-Id: I6aff58807a9c0b9b7b2f2f154af60f5103be5ac9
Reviewed-on: https://chromium-review.googlesource.com/1194953
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#587094}
2 files changed