Re: [w3c/payment-request] WIP - very rough sketch of requestShippingAddress() method (#873)

> There was some suggested changes to the state machine of the event (i.e., what happens if you call things out of order, what happens if one promise resolves before the other, etc... what happens if you requestAddress, it rejects, and but updateWith resolves? etc). Can you check if that makes sense and if we need it?

I would prefer that `requestShippingAddress()` and `requestBillingAddress()` can be called only inside of the `event.updateWith()` method. Calling the two in parallel seems to introduce a lot of complexity. Is the parallel calling designed to satisfy a requirement that I'm missing?

```javascript
// GOOD
paymentRequest.addEventListener('shippingaddressupdate', (event) => {
  event.updateWith(new Promise((updateAddressResolver) => {
    const country = paymentRequest.shippingAddress.country;
    if (country === 'US') {
      const address = await event.requestShippingAddress(['regionCode', 'country']);
      updateAddressResolver(calculatePriceForUsState(address.regionCode));
    } else {
      updateAddressResolver(calculatePriceForCountry(country));
    }
  }));
});

// BAD
paymentRequest.addEventListener('shippingaddressupdate', (event) => {
  event.updateWith(new Promise((updateAddressResolver) => {
    const country = paymentRequest.shippingAddress.country;
    updateAddressResolver(calculatePriceForCountry(country));
  }));
  const address = await event.requestShippingAddress([]);  // Too late!
});
```

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