[whatwg] registerProtocolHandler - allow site to specify more info and do custom handling

[João Eiras]

> 2. The location of an icon like a favicon.ico file or png etc.
>

This is actually a real privacy issue. The user agent would periodically fetch a remove favicon, which discloses the end user's ip.
If any, such favicon would need to be made available offline immediately when installing the protocol handler, or a data uri could be used.