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Builds out support for CTAP2.1 §5.7.5, cancelling an ongoing
enrollment. The authenticator should always return success
whether there is an enrollment or not.

Change-Id: I1a99fb111c2f00d8fd016d2211e0fba9423dc1bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636227
Commit-Queue: Matthew Webb <noviv@google.com>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667294}