[whatwg] Proposal to extend registerProtocolHandler

>From my perspective on Gmail, I would prefer to know if the user hasn't
registered because they declined previously or haven't been asked. If
they've declined previously, then calling registerProtocolHandler() in
today's UAs will not do anything. If I can't detect that state, then they'll
keep clicking and I'll keep calling and they'll get frustrated.

I'd prefer if isRegistered() was something like registeredState() and
returned REGISTERED, DECLINED, or NOTASKED. Then I could make a UI that
really reflects reality.

Michael