[NTP][Enterprise] Add attachment horizontal scroll

* Support horizontal scroll on attachments so that all attachments
  can be made visible using IntersectionObserver.
* Changed padding and margin values so that focus outline isn't
  clipped by the overflow: auto.
* Browser test waits for visibility to change and then waits for
  microtasks to insure the logic to change the classes finishes running.
  Just one or the other led to flakiness.

screencast:
https://screencast.googleplex.com/cast/NjQzNzE1MTQwMjA5ODY4OHw1MTNkNTE3ZS0yNg

Bug: 353788132
Change-Id: I2ef50608bf322f44bea490092adc8d1bcbac89dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5753885
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Roman Arora <romanarora@chromium.org>
Commit-Queue: Riley Tatum <rtatum@google.com>
Cr-Commit-Position: refs/heads/main@{#1336675}