"Get Computed Role" should reference WAI-ARIA Computed Role

[spectranaut]

Currently, GetComputedRole spec says:

Let role be the result of computing the [WAI-ARIA role](https://w3c.github.io/webdriver/#dfn-wai-aria-role) of element.

In the ARIA spec currently, we don't specify how to "compute the WAI-ARIA role", and the raw role string (i.e. "foo" for role="foo") should not be return in some cases. For example, we have synonyms (role=image and role=img) which should both return the same roles.

We are working on a fix in ARIA, and when that lands, the GetComputedRole should reference it: w3c/core-aam#167