[Web Payments] Add data attribution string to dialog

components/payments/core/payment_prefs.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 "components/payments/core/payment_prefs.h"
+
+#include "components/pref_registry/pref_registry_syncable.h"
+
+namespace payments {
+
+const char kPaymentsFirstTransactionCompleted[] =
+    "payments.first_transaction_completed";
+
+void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry) {
+  registry->RegisterBooleanPref(kPaymentsFirstTransactionCompleted, false);
+}
+
+}  // namespace payments