[ios] Password InfobarModal UI Polishing

- Never For This Site button has no longer a bold font.
- Removes the separator between Save and Never For This Site cells.
- Adds highlighting to these 2 buttons by making the Cell button
UIButtonTypeSystem.

Bug: 945478
Change-Id: If39e270a06a87d9631b9604946e8a2a91d3251bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627240
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663048}