fido/cros: support legacy U2F/HID authenticator credential creation

Change the CrOS FidoDiscovery and MakeCredentialRequestHandler to
instantiate the platform authenticator for *cross*-platform-attachment,
UP-only credential creation requests if the
DeviceSecondFactorAuthentication enterprise policy is enabled.

Such requests will enroll a legacy U2F credential and require a power
button press. This is for backwards compatibility with the U2f/HID
authenticator controlled by the same enterprise policy. That
authenticator is being disabled for WebAuthn requests in this CL by
ignoring its VID/PID during discovery.

Bug: 1159526
Change-Id: Ie24a3d035d9a2e3a2149aff11289dc0f26ff4781
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643040
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#847757}