Don't send display items to payment handler

[marcoscaceres]

When making a purchase "in real life", it is uncommon to tell the payment processor exactly what is being purchased - instead, only the merchant's info and how much it costs is shared. Otherwise, the payment processor (e.g., my local bank) could gather an extensive list of things that I've purchased over time (which is fairly scary from a privacy perspective).

Thus, in the interest of privacy, perhaps we shouldn't send the displayItems to the payment handler's service worker? Alternatively, we could instead allow the user to not share display items, or allow them to send a generic "Goods or Services" instead item instead.

Rather, when the user goes to make a purchase, where the user chooses which PWA to use to fulfill the payment, the browser can display the items there instead.

Having said that, looking over PayPal invoices received over email, PayPal does get sent the display items during a purchase. 🤔

[marcoscaceres]

duplicate of #91