AX: `aria-roledescription` does not get communicated inside contenteditable

• Component: Accessibility
• Status: RESOLVED INVALID

[Ben Cronin]

Steps: 
1. Open page at http://joycezhu.github.io/aria-roledescription.html.
2. With VoiceOver running, navigate to link in non-editable node. Note that it is read out as "GitHub link, Element with different role in non-editable node."
3. Now, navigate to the beginning of the link in the editable node and move the caret into the link.

Expected:
There is some readout communicating that we are inside "GitHub link", per its aria-roledescription.

Observed:
There is no mention of the "GitHub link" for this element (except sometimes in a very specific case of having the caret at the end of the link before a newline, which is not all that informative).

`aria-roledescription` reference: https://www.w3.org/TR/wai-aria-1.2/#aria-roledescription

[Radar WebKit Bug Importer]

<rdar://problem/76108305>