To get a component that implements nsIDocumentLoaderFactory
for a given mimetype, use nsICategoryManager to find an entry
with the mimetype as its name in the category “Gecko-Content-Viewers”.
The value of the entry is the contractid of the component.
The component is a service, so use GetService, not CreateInstance to get it.
Create a blank document using the given loadgroup and given
principal. aPrincipal is allowed to be null, in which case the
new document will get the about:blank codebase principal.