[ChromeRefresh2023] Add combobox corner radii and rounded rect border

This CL replaces the use of FocusableBorder with RoundedRectBorder
in the ChromeRefresh2023 case. (See also crbug.com/1410862)

Note: Disabled state is slightly different
for the RoundedRectBorder but this will be modified after we finalize
discussions on what the disabled state of the combobox will look like.

Bug: 1400024
Change-Id: I68bb2495d3c349efc37a027924d71877e98daef1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4206664
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elaine Chien <elainechien@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1100635}