This patch updates the followings:

- Use the Digest header instead of the MI header
- Update the identifier from mi-sha256-draft2 to mi-sha256-03
- The top-proof is now encoded in standard base64, not base64url

Http-mice-03 also changes the behavior on 0-length payloads. That will
be addressed in a followup CL.

Bug: 875721
Change-Id: I4c53a5ed6c25a62685d523c7a9d4583875a940c2
Reviewed-on: https://chromium-review.googlesource.com/1180955
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585311}