Add checkmark to base-select options

This is being discussed in OpenUI here:
https://github.com/openui/open-ui/issues/863

The discussion suggested that we should use a unicode checkmark instead
of an SVG, so that's what this patch does.

It might be better to use ::marker instead of ::before, but we might
also end up with something else entirely and I think that reusing
::marker would be harder to implement, so I am using ::before for now.

Bug: 1511354
Change-Id: I29c1dfc26208e6fc13cf42782a3d9d6fae2e711c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5578818


Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313329}