Incremental updates via updateWith()

## Incremental updates

Click on each button in sequence from top to bottom without refreshing the page. Each button will bring up the Payment Request UI window.

Unless stated otherwise, each test will update some part of the displayed payment sheet in a manner indicated below. When prompted, please change or enter a new shipping address, look for the tested change, and complete the payment.

If the payment request locks up or otherwise aborts, the test has failed.

### Incremental updates via PaymentDetailsUpdate.total

### Incremental updates via PaymentDetailsBase.displayItems

### Incremental updates via PaymentDetailsBase.shippingOptions

### Incremental updates via PaymentDetailsBase.modifiers

If you find a buggy test, please file a bug and tag one of the suggested reviewers.