PaymentRequest: The currencySystem should be non-nullable.

components/payments/payment_request.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [JavaPackage="org.chromium.payments.mojom"]
 module payments.mojom;
 
 // The shipping address that the browser process provides to the renderer
 // process. Built either by the browser or a payment app.
 struct PaymentAddress {
@@ skipping 30 matching lines @@
   string currency;
 
   // ISO 20022 CurrencyAnd30Amount. Up to 30 total digits. Up to 10 fraction
   // digits. Separated by a dot.
   string value;
 
   // currency_system is a URL that indicates the currency system that the
   // currency identifier belongs to. By default, the value is
   // urn:iso:std:iso:4217 indicating that currency is defined by [ISO4217]
   // (for example, USD for US Dollars).
-  string? currency_system;
+  string currency_system = "urn:iso:std:iso:4217";
 };
 
 struct PaymentResponse {
   string method_name;
 
   // Payment method specific JSON string that is built either by the browser or
   // a payment app, for example Android Pay. Browser ensures that the string can
   // be successfully parsed into base::JSONParser. Renderer parses this string
   // via v8::JSON::Parse() and hands off the result to the merchant website.
   // There's no one format for this object, so more specific types cannot be
@@ skipping 152 matching lines @@
   Init(PaymentRequestClient client,
        array<PaymentMethodData> method_data,
        PaymentDetails details,
        PaymentOptions options);
   Show();
   UpdateWith(PaymentDetails details);
   Abort();
   Complete(PaymentComplete result);
   CanMakePayment();
 };