[Payment Request] Refactors the remaining selector view controllers

ios/chrome/browser/payments/payment_request_selector_view_controller.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 IOS_CHROME_BROWSER_PAYMENTS_PAYMENT_REQUEST_SELECTOR_VIEW_CONTROLLER_H_
 #define IOS_CHROME_BROWSER_PAYMENTS_PAYMENT_REQUEST_SELECTOR_VIEW_CONTROLLER_H_
 
 #import <UIKit/UIKit.h>
 
 #import "ios/chrome/browser/ui/collection_view/collection_view_controller.h"
 
 @class PaymentRequestSelectorViewController;
 @protocol PaymentRequestSelectorViewControllerDataSource;
 
 // Delegate protocol for PaymentRequestSelectorViewController.
 @protocol PaymentRequestSelectorViewControllerDelegate<NSObject>
 
 // Notifies the delegate that the user has selected an item at the given index.
 - (void)paymentRequestSelectorViewController:
             (PaymentRequestSelectorViewController*)controller
                         didSelectItemAtIndex:(NSUInteger)index;
 
-// Notifies the delegate that the user has chosen to add an item.
-- (void)paymentRequestSelectorViewControllerDidSelectAddItem:
-    (PaymentRequestSelectorViewController*)controller;
-
 // Notifies the delegate that the user has chosen to return to the previous
 // screen without making a selection.
 - (void)paymentRequestSelectorViewControllerDidFinish:
     (PaymentRequestSelectorViewController*)controller;
 
+@optional
+
+// Notifies the delegate that the user has chosen to add an item.
+- (void)paymentRequestSelectorViewControllerDidSelectAddItem:
+    (PaymentRequestSelectorViewController*)controller;
+
 @end
 
 // View controller responsible for presenting a list of items provided by the
 // supplied data source for selection by the user and communicating the choice
 // to the supplied delegate. It displays an optional header item provided by the
 // data source above the list of selectable items. The list is followed by an
 // optional button to add an item.
 @interface PaymentRequestSelectorViewController : CollectionViewController
 
 // The delegate to be notified when the user selects an item, returns without
 // selection, or decides to add an item.
 @property(nonatomic, weak) id<PaymentRequestSelectorViewControllerDelegate>
     delegate;
 
 // The data source for this view controller.
 @property(nonatomic, weak) id<PaymentRequestSelectorViewControllerDataSource>
     dataSource;
 
 // Convenience initializer. Initializes this object with the
 // CollectionViewControllerStyleAppBar style and sets up the leading (back)
 // button.
 - (instancetype)init;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_PAYMENTS_PAYMENT_REQUEST_SELECTOR_VIEW_CONTROLLER_H_