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This change mostly updates FidoTunnelDevice to add support for
state-assisted (i.e. paired) transactions. That also involves some
handshaking updates because the state information signs the claimed
public key and so the handshake needs to export a binding value to sign
over.

BUG=1002262

Change-Id: Ib84886baacef24a8f55014b1a9b33617d0122d11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406834
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#807621}