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Since the U2F Zero authenticator cannot deal with check-only sign
requests, exclude lists are probed with UP-requiring sign requests
instead. Since these requests are indinstinguishable from actual sign
requests, they are problematic for authenticators that want to show the
request type to the user (e.g. soft tokens and authenticators with a
display).

This change aims to make these workaround sign requests stand out while
preserving compatibility with authenticators behaving like the U2F Zero.
This is achieved by setting the challenge of the sign requests sent
while probing an exclude list to the same fixed bogus challenge used to
collect a touch on certain errors.

R=agl@chromium.org

Bug: 1000641
Change-Id: I7ae43a0ecc2380714ce4752a84c7b6cd4572cdb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1831857
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#701648}