Add Payment Request demo content.

Add Payment Request demo content.

Rubber-stamped by Zalan Bujtas.

* demos/payment-request/demo.js: Added.
(string_appeared_here.async.applePayButtonClicked):
* demos/payment-request/index.html: Added.
* demos/payment-request/merchant-validation.php: Added.
* demos/payment-request/squirrelfish.png: Added.
* demos/payment-request/style.css: Added.
(@supports (-webkit-appearance: -apple-pay-button)):
(.apple-pay-not-supported::before):
(.hidden):
(#black-buttons > .apple-pay-button):
(#white-buttons > .apple-pay-button):
(#demo):
(#description):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@230270 268f45cc-cd09-0410-ab3c-d52691b4dbfc