Re: [w3c/browser-payment-api] What is the extensibility mechanism for the payment request and response messages? (#146)

@msporny, I think you misinterpreted my comment or perhaps I misunderstand your intent.

As someone that is not intimately familiar with how a browser processes input I have some questions. Assuming a browser API is defined that accepts the following message (described using WebIDL):

```
interface HelloApi {
  void sayHello(HelloWorld message);
};

dictionary HelloWorld {
  required DOMString title;
  required DOMString message;
};
```
And a website calls that API with the following code:
```javascript
//Assume hello is an instance of HelloApi...
hello.sayHello({
  title: "Hello",
  message: "World",
  otherThing: "!"
});
```
I would expect the browser to throw an error, no? If so, how do we define a fully extensible message in WebIDL when its not possible to allow for extra top-level members?

Do we simply accept a parameter of type `object` and describe the data model loosely in the messages spec? I think that the browser vendors have already said that's not a workable solution.

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