Reland "[NTP] Add ability to add/delete/reset custom links from the New Tab Page"

This is a reland of c7c6a230da8a075b37f7306f4f8902e7a3f3b490

The iframe was sometimes emitting a DOMContentLoaded event before the
src was modified.

Original change's description:
> [NTP] Add ability to add/delete/reset custom links from the New Tab Page
>
> Added functionality for adding/removing links and restoring the Most
> Visited tiles. If the URL is not valid, the link will not be added. The
> blacklist button now opens the edit dialog (editing links is currently
> not functional and will be added in a later CL).
>
> Also added hover/pressed/focus styling to the input fields and the
> buttons.
>
> Design Doc: go/chrome-ntp-custom-links
>
> Video: https://screencast.googleplex.com/cast/NTQyNDU2OTc1ODI1MzA1NnwwMjFkODAwYi1kMg
>
> Bug: 851293
> Change-Id: Ic92e46ba93a4abcb375467a1d047cbb573f231ba
> Reviewed-on: https://chromium-review.googlesource.com/1149458
> Commit-Queue: Kristi Park <[email protected]>
> Reviewed-by: Marc Treib <[email protected]>
> Reviewed-by: Bernhard Bauer <[email protected]>
> Reviewed-by: Mathieu Perreault <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#578404}

[email protected]

Bug: 851293
Change-Id: Ibc4e29d1b5a9cedbd6151e0e02d14c6ae8eb9bd0
Reviewed-on: https://chromium-review.googlesource.com/1153314
Commit-Queue: Kristi Park <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Cr-Commit-Position: refs/heads/master@{#578989}
19 files changed