[Web Payments] Add data attribution string to dialog

components/payments/core/payment_prefs.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_CORE_PAYMENT_PREFS_H_
+#define COMPONENTS_PAYMENTS_CORE_PAYMENT_PREFS_H_
+
+namespace user_prefs {
+class PrefRegistrySyncable;
+}
+
+namespace payments {
+
+// True if the profile has already successfully completed at least one payment
+// request transaction.
+extern const char kPaymentsFirstTransactionCompleted[];
+
+void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
+
+}  // namespace payments
+
+#endif  // COMPONENTS_PAYMENTS_CORE_PAYMENT_PREFS_H_