Sanitize nameservers and improve UI

This enforces 4 valid nameserver address in the ONC -> Shill
translation. It also fixes the UI to support Google nameservers with
empty entries correctly.

Bug: 922219
Change-Id: Ib7439a0a4010ee574e60f30f4c676a2379800328
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607508
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661433}
6 files changed