This interface should be implemented by an XPCOM factory that
is registered to handle “@mozilla.org/rdf/delegate-factory/[key]/[scheme];1”
ContractIDs.
The factory will be invoked to create delegate objects from
nsIRDFResource::GetDelegate().
Create a delegate for the specified RDF resource.
The created delegate should forward AddRef() and Release()
calls to the aOuter object.