Use the single char overload of append() when possible.

third_party/WebKit/Source/modules/payments/PaymentAddress.cpp

 // 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.
 
 #include "modules/payments/PaymentAddress.h"
 
+#include "wtf/text/StringBuilder.h"
+
 namespace blink {
 
 PaymentAddress::PaymentAddress(mojom::blink::PaymentAddressPtr address)
     : m_country(address->country)
     , m_addressLine(address->address_line.PassStorage())
     , m_region(address->region)
     , m_city(address->city)
     , m_dependentLocality(address->dependent_locality)
     , m_postalCode(address->postal_code)
     , m_sortingCode(address->sorting_code)
     , m_languageCode(address->language_code)
     , m_organization(address->organization)
     , m_recipient(address->recipient)
     , m_careOf(address->careOf)
     , m_phone(address->phone)
 {
     if (!m_languageCode.isEmpty() && !address->script_code.isEmpty()) {
-        m_languageCode.append("-");
-        m_languageCode.append(address->script_code);
+        StringBuilder builder;
+        builder.append(m_languageCode);
+        builder.append('-');
+        builder.append(address->script_code);
+        m_languageCode = builder.toString();
     }
 }
 
 PaymentAddress::~PaymentAddress() {}
 
 } // namespace blink