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This change allows VirtualU2fDevice to create custom attestation
certificates at run-time. It also adds tests for attestation behaviour
that depend on that ability.

Change-Id: I88323de6baaba7883ab0db676c26ccdb4b0dafbb
Reviewed-on: https://chromium-review.googlesource.com/967066Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544970}