This is enough for Firefox to display the Accessibility tab, containing
our accessibility tree which can be inspected. Most information is
blank for now.
There's quite a bit of duplication between AccessibilityWalkerActor and
WalkerActor - it might be worth trying to make a base class once the
details are figured out. Frustratingly, the two don't work quite the
same: for a lot of messages that would be sent to WalkerActor, the
accessibility equivalent is sent to the AccessibilityNodeActor instead.
Co-authored-by: Tim Flynn <trflynn89@pm.me>
