gen-certurl's -sct option is deprecated in
https://github.com/WICG/webpackage/pull/259.

Bug: 803774
Change-Id: I4fbc575366aba978b262418d2cf415121e840099
Reviewed-on: https://chromium-review.googlesource.com/1154745
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578999}