To match spec text changes in https://github.com/WICG/webpackage/commit/3cae549.

Bug: 803774
Change-Id: I31dc56506cdaf1d67b05eba15b77d3c7fef89b33
Reviewed-on: https://chromium-review.googlesource.com/958748
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542450}