Signed Exchange: Append "-draft2" to MI encoding identifiers

Bug: 803774
Change-Id: I91b9def8e3e4506c4d8913cb351129286f3686d1
Reviewed-on: https://chromium-review.googlesource.com/1141668
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576300}
14 files changed