[NTP] Change custom bg attribution to an <a href...> tag

This gives the attribution link the proper context menu.

Screencast: http://go/scrcast/NTkxNDI0NDY2MDg1NDc4NHwyOGZhZTViMy1jMQ

Bug: 963399
Change-Id: Iadccc1a9457b3c7ac6f1093197c937190815821b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616998
Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661054}
3 files changed