[WebPayments] Implement state transitions in desktop WebPayments dialog.

chrome/browser/ui/views/payments/order_summary_state_observer.h

+// 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.
+
+#ifndef CHROME_BROWSER_UI_VIEWS_PAYMENTS_ORDER_SUMMARY_STATE_OBSERVER_H_
+#define CHROME_BROWSER_UI_VIEWS_PAYMENTS_ORDER_SUMMARY_STATE_OBSERVER_H_
+
+namespace payments {
+
+class OrderSummaryStateObserver {
+ public:
+  virtual void OnBackClicked() {}
+
+ protected:
+  virtual ~OrderSummaryStateObserver() {}
+};
+
+}  // namespace payments
+
+#endif  // CHROME_BROWSER_UI_VIEWS_PAYMENTS_ORDER_SUMMARY_STATE_OBSERVER_H_