Link rel=modulepreload: Add comments that map code to the spec

This patch also fixes the followings:
- Reject modulepreloads with invalid as="" value
- ParserState should be "not parser inserted"

Bug: 740886

Change-Id: I9f3c8e87da31a0b09bad208d681e200b6a06d69e
Reviewed-on: https://chromium-review.googlesource.com/768773
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516974}
4 files changed