Re: [w3c/payment-request] Merchant validation (#646)

@aestes, I would prefer to make this method generic, so that payment handlers have the option of firing the `merchantvalidation` event as well. The code in the payment handler would look like this:

```javascript
const VALIDATION_URL = '...';
const SESSION_ID = '...';
self.addEventListener('canmakepayment', (c) => {
  c.validateMerchant(VALIDATION_URL).then((b) => {
    c.respondWith(b == SESSION_ID);
  });
});
```

The code on the merchant website would look like this:

```javascript

const MERCHANT_SERVER = '...';
paymentRequest.addEventListener('merchantvalidation', (v) => {
  v.complete(new Promise(async resolve => {
    const b = await fetch(MERCHANT_SERVER +
        encodeURIComponent(v.validationUrl)).then(r => r.text());
    resolve(b);
  });
});
```

The overall flow would work like this:

![Generic merchant validation](https://i.imgur.com/6EzrU8f.png)

What do you think? I'm open to suggestions and persuasion: we don't *have* to have a generic merchant validation mechanism if you feel that's right.

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