[WebPayments] Implement state transitions in desktop WebPayments dialog.

chrome/browser/ui/views/payments/payment_request_dialog.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_PAYMENT_REQUEST_DIALOG_H_
 #define CHROME_BROWSER_UI_VIEWS_PAYMENTS_PAYMENT_REQUEST_DIALOG_H_
 
-#include "ui/views/controls/label.h"
+#include "chrome/browser/ui/views/payments/order_summary_state_observer.h"
+#include "chrome/browser/ui/views/payments/payment_sheet_state_observer.h"
 #include "ui/views/window/dialog_delegate.h"
 
+class ViewStack;
+
 namespace payments {
 
 class PaymentRequestImpl;
 
 // The dialog delegate that represents a desktop WebPayments dialog. This class
 // is responsible for displaying the view associated with the current state of
 // the WebPayments flow and managing the transition between those states.
-class PaymentRequestDialog : public views::DialogDelegateView {
+class PaymentRequestDialog : public views::DialogDelegateView,
+                             public PaymentSheetStateObserver,
+                             public OrderSummaryStateObserver {
  public:
   explicit PaymentRequestDialog(PaymentRequestImpl* impl);
   ~PaymentRequestDialog() override;
 
-  // views::WidgetDelegate:
+  // views::WidgetDelegate
   ui::ModalType GetModalType() const override;
 
-  // views::View:
+  // views::View
   gfx::Size GetPreferredSize() const override;
 
-  // views::DialogDelegate:
+  // views::DialogDelegate
   bool Cancel() override;
 
+  // PaymentSheetStateObserver:
+  void OnOrderSummaryClicked() override;
+
+  // OrderSummaryStateObserver:
+  void OnBackClicked() override;
+
  private:
   // Non-owned reference to the PaymentRequestImpl that initiated this dialog.
   // Since the PaymentRequestImpl object always outlives this one, the pointer
   // should always be valid even though there is no direct ownership
   // relationship between the two.
   PaymentRequestImpl* impl_;
-  std::unique_ptr<views::Label> label_;
+  std::unique_ptr<ViewStack> view_stack_;
 
   DISALLOW_COPY_AND_ASSIGN(PaymentRequestDialog);
 };
 
 }  // namespace payments
 
 #endif  // CHROME_BROWSER_UI_VIEWS_PAYMENTS_PAYMENT_REQUEST_DIALOG_H_