Re: [w3c/browser-payment-api] canMakePayment() might be in the wrong place? (#403)

I'm not opposed to instantiating `PaymentRequest` with dummy data to find out whether `.canMakePayment()` will return `true`.

```javascript
// The page has loaded. Should the page use PaymentRequest?
// If PaymentRequest fails, should the page fallback to manual
// web form checkout?
const supportedPaymentMethods = ...
let shouldCallPaymentRequest = true;
let fallbackToLegacyOnPaymentRequestFailure = false;
(new PaymentRequest(supportedPaymentMethods,
    {total: {label: 'Stub', amount: {currency: 'USD', value: '0.01'}}})
.canMakePayment()
.then(function(result) {
  shouldCallPaymentRequest = result;
}).catch(function(error) {
  console.log(error);
  // The user may have turned off query ability in their privacy settings.
  // Let's use PaymentRequest by default and fallback to legacy
  // web form based checkout.
  shouldCallPaymentRequest = true;
  fallbackToLegacyOnPaymentRequestFailure = true;
});

// User has clicked on the checkout button. We know
// what's in the cart, but we don't have a `Checkout` object.
function onCheckoutButtonClicked(lineItems) {
  callServerToRetrieveCheckoutDetails(lineItems);
}

// The server has constructed the `Checkout` object. Now we know
// all of the prices and shipping options.
function onServerCheckoutDetailsRetrieved(checkoutObject) {
  if (shouldCallPaymentRequest) {
    const request = new PaymentRequest(supportedPaymentMethods, checkoutObject);
    request.show().then(function(paymentResponse) {
      // Post the results to the server and call `paymeResponse.complete()`.
    }).catch(function(error) {
      console.log(error);
      if (fallbackToLegacyOnPaymentRequestFailure) {
        window.location.href = '/legacy-web-form-checkout';
      } else {
        showCheckoutErrorToUser();
      }
    });
  } else {
    window.location.href = '/legacy-web-form-checkout';
  }
}
```

If you call `.canMakePayment()` multiple times, keep in mind that the first parameter to the `PaymentRequest` constructor should contain the same data

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