This revision is a step towards ensuring we get the right domains for known
accessibility instantiators. We want to anticipate the needs of known clients
and make sure we have all the cache domains ready, especially for heavy-use
clients like screen readers. The domains we'll need are a bit of a work in
progress, but can be updated easily. For now, err on the side of caution. This
revision also implements better client detection on macOS, so we can handle
multiple clients.
Differential Revision: https://phabricator.services.mozilla.com/D220035
