[iOS][Alert] Create buttons with gray highlight

Adds a button that changes its background to gray when highlighted. This
is to be used in the buttons of the new AlertViewController.

Bug: 951358, 951300
Change-Id: Ib76fb1eef38f911de096781050b101997f091b70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578743
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653550}
3 files changed