[Payments] Add an entry for Payment Request to chrome://flags on iOS

components/payments/core/features.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_FEATURES_H_
+#define COMPONENTS_PAYMENTS_CORE_FEATURES_H_
+
+#include "base/feature_list.h"
+#include "build/build_config.h"
+
+namespace payments {
+namespace features {
+
+#if defined(OS_IOS)
+// Used to control the state of the Payment Request API feature.
+extern const base::Feature kWebPayments;
+#endif
+
+}  // namespace features
+}  // namespace payments
+
+#endif  // COMPONENTS_PAYMENTS_CORE_FEATURES_H_