Add URL display guidelines to docs/security

This is a public version of our internal guidance for how to build
secure URL displays.

Change-Id: Icb6d039cea19b1d07e40766b1c7b503b3bbeaabf
Reviewed-on: https://chromium-review.googlesource.com/c/1402344
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Christopher Thompson <cthomp@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621828}
16 files changed