Incorrect mapping for aria-description in AX API

[aleventhal]

According to https://bugs.webkit.org/show_bug.cgi?id=206253#c18
AXCustomContent should be used if MacOS version >= 11
Otherwise, use AXHelp (as it says now).

[cookiecrook]

Commenting that previous PR and branch were incomplete so I've deleted them. Waiting for full implementation before completing the AAM diff. Sorry for the false start, Aaron.

[aleventhal]

Thanks @cookiecrook. When you say waiting for full implementation, does that mean you will wait until stable versions of MacOS and Safari are released? In general it's helpful for us on the Chrome team to have mappings as early as possible. Is there a reason to wait before publishing what the mapping will be? IMO we can consider this a living document.