[WebPayments] Implement state transitions in desktop WebPayments dialog.

chrome/browser/payments/ui/order_summary_state.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_PAYMENTS_UI_ORDER_SUMMARY_STATE_H_
+#define CHROME_BROWSER_PAYMENTS_UI_ORDER_SUMMARY_STATE_H_
+
+#include "base/macros.h"
+#include "chrome/browser/payments/ui/payment_dialog_state.h"
+#include "ui/views/controls/button/button.h"
+
+namespace views {
+class View;
+}
+
+namespace payments {
+
+// Encapsulates view creation and event handling for the Order Summary screen of
+// the desktop WebPayments UI.
+class OrderSummaryState : public PaymentDialogState,
+                          public views::ButtonListener {
+ public:
+  class Observer {
+   public:
+    virtual void OnBackClicked() {}
+  };
+
+  OrderSummaryState();
+  ~OrderSummaryState() override;
+
+  // PaymentDialogState:
+  views::View* GetView() override;
+
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+ private:
+  void ButtonPressed(views::Button* sender, const ui::Event& event) override;
+
+  views::View* view_;
+  base::ObserverList<Observer> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(OrderSummaryState);
+};
+
+}  // namespace payments
+
+#endif  // CHROME_BROWSER_PAYMENTS_UI_ORDER_SUMMARY_STATE_H_