PaymentRequest: The currencySystem should be non-nullable.

components/payments/currency_formatter.h

 // Copyright 2017 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.
 
 #ifndef COMPONENTS_PAYMENTS_CURRENCY_FORMATTER_H_
 #define COMPONENTS_PAYMENTS_CURRENCY_FORMATTER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 11 matching lines @@
 
 // Currency formatter for amounts, according to a currency code, which typically
 // adheres to [ISO4217] (for example, "USD" for US Dollars).
 class CurrencyFormatter {
  public:
   // Initializes the CurrencyFormatter for a given |currency_code|,
   // |currency_system| and |locale_name|. Note that |currency_code| and
   // |currency_system| should have been validated (as part of
   // payment_details_validation.h) before this is created.
   CurrencyFormatter(const std::string& currency_code,
-                    const base::Optional<std::string> currency_system,
+                    const std::string& currency_system,
                     const std::string& locale_name);
   ~CurrencyFormatter();
 
   // Formats the |amount| according to the currency code that was set. The
   // result will NOT contain the currency code, nor a subset of it. Rather, the
   // caller of this function should display the currency code separately. The
   // return value may contain non-breaking space and is ready for display. In
   // the case of a failure in initialization of the formatter or during
   // formatter, this method will return |amount|.
   base::string16 Format(const std::string& amount);
 
   // Returns the formatted currency code (<= 6 characters including ellipsis if
   // applicable).
   std::string formatted_currency_code() { return formatted_currency_code_; }
 
  private:
   const icu::Locale locale_;
   std::unique_ptr<icu::UnicodeString> currency_code_;
   std::string formatted_currency_code_;
   std::unique_ptr<icu::NumberFormat> icu_formatter_;
 
   DISALLOW_COPY_AND_ASSIGN(CurrencyFormatter);
 };
 
 }  // namespace payments
 
 #endif  // COMPONENTS_PAYMENTS_CURRENCY_FORMATTER_H_