Expose border-radius for cr-action-menu item

Expose the border radius as a CSS variable to enable updating it.

Bug: b:281136249
Change-Id: I81d12443f25e4eb31ce403f51a4a58c2c1b24e68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4554663


Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148283}