WebCore::PaymentShippingOption::selected value is ignored

[Antoine du Hamel]

According to https://www.w3.org/TR/payment-request/#paymentshippingoption-dictionary, setting the selected member to true indicates that this is the default selected PaymentShippingOption in a sequence. User agents SHOULD display this option by default in the user interface.

It seems WebKit always pick up the first `PaymentShippingOption` in the sequence, and ignores the `selected` member.

```js
const currency = 'USD';
const shippingOptions = [
                {
                  id: "fancy-postal",
                  label: "Fancy postal service",
                  amount: { currency, value: "100" },
                },
                {
                  id: "basic-postal",
                  label: "Default postal service",
                  amount: { currency, value: "1.00" },
                  selected: true,
                },
                {
                  id: "free-postal",
                  label: "Free postal service",
                  amount: { currency, value: "0" },
                },
];
```

In this example, `basic-postal` should be the default choice (which is the case on Chromium), but instead `fancy-postal` is selected by default.

Demo available here: https://aduh95.github.io/payment-request-test/

[Radar WebKit Bug Importer]

<rdar://problem/50193130>