[Payments] Add combobox support to editors.

chrome/browser/ui/views/payments/preselected_combobox_model.cc

+// 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.
+
+#include "chrome/browser/ui/views/payments/preselected_combobox_model.h"
+
+namespace payments {
+
+PreselectedComboboxModel::PreselectedComboboxModel(
+    const std::vector<base::string16>& items,
+    int default_index)
+    : SimpleComboboxModel(items), default_index_(default_index) {}
+
+PreselectedComboboxModel::~PreselectedComboboxModel() {}
+
+int PreselectedComboboxModel::GetDefaultIndex() const {
+  return default_index_;
+}
+
+}  // namespace payments