Disambiguate "parameters" in Set Permission steps

[saschanaz]

https://w3c.github.io/permissions/#webdriver-command-set-permission

1. Let parameters be the parameters argument, converted to an IDL value of type PermissionSetParameters. If this throws an exception, return an invalid argument error.
2. Let rootDesc be parameters.descriptor.

But it's not clear what the "parameters" in the second step refers to. It should be the argument, or otherwise rootDesc won't have extra members from e.g. MidiPermissionDescriptor. That should be made clear.

The Streams spec uses Dict to disambiguate in a similar case: https://streams.spec.whatwg.org/#rs-constructor

1. Let underlyingSourceDict be underlyingSource, converted to an IDL value of type UnderlyingSource.

We should probably follow that here.