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In preparation to add support for the new cert format, this patch
introduces SignedExchangeCertificateChain class that contains all
information of a certificate chain. Certificate parsing code in
SignedExchangeCertFetcher is moved to this new class.

Pure refactoring, no behavior change.

Bug: 815024,815025
Change-Id: Iacb592279c9fef7afb40cb303ef81eebb4be34a3
Reviewed-on: https://chromium-review.googlesource.com/1002339Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549400}