feat: add Google Pay through Braintree

To use Google Pay through Braintree, ensure that you have enabled
Google Pay on a Braintree gateway in the Recurly Admin UI and on your
Braintree account.

Then, initialize `recurly.GooglePay(...)` with the
`braintree.clientAuthorization` option:

```js
const googlePay = recurly.GooglePay({
  ...options,
  gatewayCode: '<Recurly Braintree gateway code>',
  braintree: {
    clientAuthorization: '<Braintree client authorization key>',
  },
})
```

lib/recurly/google-pay/google-pay.braintree.js

@@ -0,0 +1,64 @@
+import Promise from 'promise';
+import { GooglePay } from './google-pay';
+import Debug from 'debug';
+import BraintreeLoader from '../../util/braintree-loader';
+import errors from '../errors';
+import { payWithGoogle } from './pay-with-google';
+
+const debug = Debug('recurly:google-pay:braintree');
+
+export class GooglePayBraintree extends GooglePay {
+  configure (options) {
+    debug('Initializing client');
+
+    const authorization = options.braintree.clientAuthorization;
+
+    BraintreeLoader.loadModules('googlePayment', 'dataCollector')
+      .then(() => window.braintree.client.create({ authorization }))
+      .then(client => Promise.all([
+        window.braintree.dataCollector.create({ client }),
+        window.braintree.googlePayment.create({
+          client,
+          googlePayVersion: 2,
+          googleMerchantId: options.googleMerchantId,
+        })
+      ]))
+      .then(([dataCollector, googlePayment]) => { this.braintree = { dataCollector, googlePayment }; })
+      .catch(err => this.emit('error', errors('google-pay-init-error', { err })))
+      .then(() => super.configure(options));
+  }
+
+  createButton ({ paymentOptions, isReadyToPayRequest, paymentDataRequest: recurlyPaymentDataRequest, buttonOptions }) {
+    // allow Braintree to set the tokenizationSpecification for its gateway
+    recurlyPaymentDataRequest.allowedPaymentMethods.forEach(method => {
+      if (method.tokenizationSpecification) delete method.tokenizationSpecification;
+    });
+    const paymentDataRequest = this.braintree.googlePayment.createPaymentDataRequest(recurlyPaymentDataRequest);
+    debug('Creating button', recurlyPaymentDataRequest, paymentDataRequest);
+    return payWithGoogle({
+      paymentOptions,
+      isReadyToPayRequest,
+      paymentDataRequest,
+      buttonOptions,
+    });
+  }
+
+  token (paymentData) {
+    debug('Creating token', paymentData);
+
+    return this.braintree.googlePayment
+      .parseResponse(paymentData)
+      .then(token => {
+        token.deviceData = this.braintree.dataCollector.deviceData;
+        paymentData.paymentMethodData.gatewayToken = token;
+        return super.token(paymentData);
+      });
+  }
+
+  mapPaymentData (paymentData) {
+    return {
+      type: 'braintree',
+      ...super.mapPaymentData(paymentData),
+    };
+  }
+}

lib/recurly/google-pay/index.js

@@ -1,4 +1,5 @@
 import { GooglePay } from './google-pay';
+import { GooglePayBraintree } from './google-pay.braintree';
 
 /**
  * Returns a GooglePay instance.
@@ -7,7 +8,9 @@ import { GooglePay } from './google-pay';
  * @return {GooglePay}
  */
 export function factory (options) {
-  const factoryClass = GooglePay;
+  const factoryClass = options?.braintree?.clientAuthorization
+    ? GooglePayBraintree
+    : GooglePay;
 
   return new factoryClass(Object.assign({}, options, { recurly: this }));
 }