Adding accessibility to the close button in sign-in promo

ios/chrome/browser/ui/bookmarks/bookmark_signin_promo_cell.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_UI_BOOKMARKS_BOOKMARK_SIGNIN_PROMO_CELL_H_
 #define IOS_CHROME_BROWSER_UI_BOOKMARKS_BOOKMARK_SIGNIN_PROMO_CELL_H_
 
 #import <UIKit/UIKit.h>
 
 @class SigninPromoView;
 
 typedef void (^CloseButtonCallback)(void);
 
 // Sign-in promo cell based on SigninPromoView. This cell invites the user to
 // login without typing their password.
 @interface BookmarkSigninPromoCell : UICollectionViewCell
 
 // Identifier for -[UICollectionView registerClass:forCellWithReuseIdentifier:].
 + (NSString*)reuseIdentifier;
 
 @property(nonatomic, readonly) SigninPromoView* signinPromoView;
+// Called when the user taps on the close button. If not set, the close button
+// is hidden.
 @property(nonatomic, copy) CloseButtonCallback closeButtonAction;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_BOOKMARKS_BOOKMARK_SIGNIN_PROMO_CELL_H_