I don't think we want type converters for these types if we can avoid them.
We have StructTraits for mapping directly to the url mojom wire format.

Hi, Ken, Thanks a lot for so quick feedback! But when I want to transfer mojom::CredentialInfo via a mojo interface function, seems I must do the conversion between url::Origin and url::mojom::OriginPtr. 
I suppose the StructTraits is for the serialization/deserialization work inside mojo and how could mojo interface function callers use StructTraits directly? Sorry maybe I'm misunderstanding about StructTraits.. Thanks in advance~

Generated class:
mojom::CredentialInfo {
   XXXXXX
   url::mojom::OriginPtr federation;
   XXXXXX
}

What you want to do is apply a typemap to your mojom. So your mojom rule
would look something like:

mojom("interfaces") {
  sources = [
    "credential_manager.mojom"
  ]
  typemaps = [
    "//url/mojo/origin.typemap"
  ]
}

If you do this, your generated bindings will replace references to
url.mojom.Origin with references to url::Origin, so your example generated
struct would look like:

class Credentalinfo {
 // ...
 url::Origin federation;
};

and the bindings will understand how to serialize the |federation| field in
the wire format defined by url::mojom::Origin. This also works if you use
url.mojom.Origin as a message parameter. The method signature will expect a
const url::Origin& instead of a url::mojom::OriginPtr.

Big Thanks! It's clear now and I think this is amazing! Thanks and sorry to take your time for this issue!

And, sorry for additional question: 
Seems we're using type_converters in many mojofication works until now, and now the typemap/StructTraits seems more direct and powerful than type_converters, should we prefer using typemap/StructTraits for all defined mojom struct in future mojofication work. Or is there any criteria to help make a choice? Thanks~