WebUI: cr-checkbox, hide button inside checkbox from screenreaders

Bug: 870445
Change-Id: I9773698a1883f32b39adee9de7b1e7cfc2325c15
Reviewed-on: https://chromium-review.googlesource.com/c/1401455
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622253}
1 file changed