[A11Y] Adding W3C ARIA APG as a third_party dependency

As part of the broader effort to audit and add automated testing for
ARIA web control patterns, we need a reliable way to pull in the
official W3C examples.

Currently, test files are copied and stripped down manually. This change
adds W3C ARIA Practices GitHub repository
(https://github.com/w3c/aria-practices) as a third_party dependency.
This will ensure that our tests remain spec-compliant and increase the
visibility of upstream bug fixes.

Chrome ATLs approval -
https://groups.google.com/a/google.com/g/chrome-atls-discuss/c/P2nP7pR1TMg/m/X3jZsn3VBAAJ

Bug: 507020351
Change-Id: Ia42574a3d0f04ec6eccd01222c45b679040f17a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7835970
Reviewed-by: Kyra Seevers <kyraseevers@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Aditya Batra <adibatra@google.com>
Cr-Commit-Position: refs/heads/main@{#1631353}