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After this patch:
- SignedExchangeHeader parses the Signature: header
- SignedExchangeSignatureVerifier takes a SignedExchangeHeader instead
  of custom input parameter struct
- SignedExchangeHandler holds SignedExchangeHeader as a member

Bug: 803774
Change-Id: I842f211360c93b11b52a495f6b0c6fa5b664c197
Reviewed-on: https://chromium-review.googlesource.com/954904
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542424}