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This is a follow-up to crrev.com/c/981813.
- Update spec links to the implementation checkpoint
- Use #section-name instead of #rfc.section.numbers as numbers may change
- Remove unused constants from signed_exchange_consts.h

Bug: 803774
Change-Id: I460d580f30275c6a586cee4b1c7540ff80a98791
Reviewed-on: https://chromium-review.googlesource.com/981874Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546056}