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This adds a parser for the CBOR certificate chain format defined in [1].
SignedExchangeCertificateChain::Parse() takes a version enum and selects
a parser to use. For now, the new parser is used only by tests.

[1] https://wicg.github.io/webpackage/draft-yasskin-http-origin-signed-responses.html#cert-chain-format

Bug: 815024,815025
Change-Id: Ia554ad3d086dbecd20294bdb6db03f37b60d67d9
Reviewed-on: https://chromium-review.googlesource.com/1002412
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558182}