[w3c/browser-payment-api] Initial values for internal slots (#436)

The spec says:

```HTML
The <a>PaymentRequestUpdateEvent</a> constructor MUST set the
internal slot [[\waitForUpdate]] to false.
```

For things like the above, might be better for the above just to have initial values for internal slots. 

It's also a bit odd to declare something related to a calling the constructor outside of an algorithm. 



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