Add web-payments sample

Author: rsolomakhin

common/input/web-payment/index.html

@@ -0,0 +1,14 @@
+---
+layout: page
+favicon: gray
+background: gray
+---
+
+<div id="content">
+  <h1>web-payment</h1>
+  <p><button onclick="handleClick();">Initiate payment</button></p>
+</div>
+
+<div id="footer">This page requires web payment API.</div>
+
+<script src="index.js"></script>

common/input/web-payment/index.js

@@ -0,0 +1,34 @@
+/**
+ * Builds PaymentRequest for credit cards, but does not show any UI yet.
+ * @return {PaymentRequest} The PaymentRequest object.
+ */
+function initPaymentRequest() {
+  return new PaymentRequest(
+      [{
+        supportedMethods: ['basic-card'],
+      }],
+      {
+        total: {
+          label: 'Total',
+          amount: {
+            currency: 'USD',
+            value: '1.00',
+          },
+        },
+      });
+}
+
+let request = initPaymentRequest();
+
+/** Invokes PaymentRequest for credit cards. */
+function handleClick() {
+  request.show()
+      .then(function(instrumentResponse) {
+        return instrumentResponse.complete('success');
+      })
+      .catch(function(err) {
+        console.log(err);
+      });
+
+  request = initPaymentRequest();
+}