navigator.isProtocolHandlerRegistered and .unregisterProtocolHandler

[c960657]

In addition to navigator.registerProtocolHandler I would like to see support for navigator.isProtocolHandlerRegistered and navigator.unregisterProtocolHandler.

The methods were defined in an early HTML5 draft specification but were removed in HTML 5.1.

Chrome 60
unregisterProtocolHandler is supported
isProtocolHandlerRegistered is WontFix'ed: https://bugs.chromium.org/p/chromium/issues/detail?id=121825

Firefox 55
Not supported
https://bugzilla.mozilla.org/show_bug.cgi?id=440620

Safari 10.1.2
Not spported, despite this: https://bugs.webkit.org/show_bug.cgi?id=73176