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This allows the Relying Party to request that a token's attestation
certificate be replaced with a randomly-generated certificate.

The default behavior is unchanged.

BUG=780299

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I98d6a94399ac1896b6893c8c080f874d66b818b3
Reviewed-on: https://chromium-review.googlesource.com/804978Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Matt Braithwaite <mab@google.com>
Cr-Commit-Position: refs/heads/master@{#521934}