webauthn: plumb a ReAuthentication Proof Token to the enclave.

In order to authorize certain operations, the enclave will demand a
reauthentication token from GAIA. This change plumbs that value so that
it can be sent.

(Tests for this are a couple of changes up the CL stack.)

Change-Id: I23ba7eae590eb2e02f37fab4687138b7ff7355c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407362


Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282192}