Add a few missing type annotations to braintree externs

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156148567

java/google/registry/ui/externs/braintree.js

@@ -196,9 +196,15 @@ braintreepayments.Integrator.prototype.teardown = function(opt_callback) {};
 braintreepayments.PaypalIntegrator = function() {};
 
 
+/**
+ * @return {void}
+ */
 braintreepayments.PaypalIntegrator.prototype.closeAuthFlow = function() {};
 
 
+/**
+ * @return {void}
+ */
 braintreepayments.PaypalIntegrator.prototype.initAuthFlow = function() {};