Make port part of origin about webusb

[beaufortfrancois]

It may not belong to the first version of the spec but I agree that origin port could be included if we're seeing custom origins.
cc @reillyeon

[reillyeon]

URL descriptors represent URLs, not origins so the structure would have to be a little more complex:

Offset  Field             Size  Value       Description
0       bLength           1     Number      Size of this descriptor.
1       bDescriptorType   1     Constant    WEBUSB_URL.
2       bScheme           1     Number      URL scheme.
3       bPort             2     Number      URL port.
5       Host Variable           String      UTF-8 encoded host, query and fragment.

Pragmatically, most URLs will use the default port for their scheme so we save 2 bytes in the usual case at the expense of requiring 2-7 in unusual cases and avoid an awkward parsing algorithm that requires replacing the port number after the URL has already been parsed.

[reillyeon]

The current URL descriptor encoding stores the port number as part of the URL. Only the scheme is handled specially since it will be present in all URLs.