WebUI: cr-checkbox, move checkbox role to checkbox for a11y

This CL makes the default label the contents of label-container. The
label and description can be overridden by passing in label and
description strings that are bound to elements that will be visually
hidden and visible to screenreaders for use in aria-labelledby and
aria-describedby.

Bug: 870443
Change-Id: I14589a329004806365223500343a6653fd06d128
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532369
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646940}