Adding accessibility to the close button in sign-in promo

ios/chrome/browser/ui/bookmarks/bookmark_signin_promo_cell.mm

 // 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.
 
 #import "ios/chrome/browser/ui/bookmarks/bookmark_signin_promo_cell.h"
 
 #import "ios/chrome/browser/ui/authentication/signin_promo_view.h"
 #import "ios/chrome/browser/ui/uikit_ui_util.h"
 #include "ios/chrome/grit/ios_chromium_strings.h"
 #include "ui/base/l10n/l10n_util.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
-namespace {
-// Close button size.
-const CGFloat kCloseButtonSize = 24;
-}
-
 @implementation BookmarkSigninPromoCell {
   SigninPromoView* _signinPromoView;
   UIButton* _closeButton;
 }
 
 @synthesize signinPromoView = _signinPromoView;
 @synthesize closeButtonAction = _closeButtonAction;
 
 + (NSString*)reuseIdentifier {
   return @"BookmarkSigninPromoCell";
 }
 
 + (BOOL)requiresConstraintBasedLayout {
   return YES;
 }
 
 - (instancetype)initWithFrame:(CGRect)frame {
   self = [super initWithFrame:frame];
   if (self) {
     UIView* contentView = self.contentView;
     _signinPromoView = [[SigninPromoView alloc] initWithFrame:self.bounds];
     _signinPromoView.translatesAutoresizingMaskIntoConstraints = NO;
     [contentView addSubview:_signinPromoView];
     AddSameConstraints(_signinPromoView, contentView);
 
-    _closeButton = [[UIButton alloc]
-        initWithFrame:CGRectMake(0, 0, kCloseButtonSize, kCloseButtonSize)];
-    [_closeButton addTarget:self
-                     action:@selector(closeButtonAction:)
-           forControlEvents:UIControlEventTouchUpInside];
-    _closeButton.translatesAutoresizingMaskIntoConstraints = NO;
-    [contentView addSubview:_closeButton];
-    [_closeButton setImage:[UIImage imageNamed:@"signin_promo_close_gray"]
-                  forState:UIControlStateNormal];
-    NSArray* buttonVisualConstraints =
-        @[ @"H:[closeButton]-|", @"V:|-[closeButton]" ];
-    NSDictionary* views = @{ @"closeButton" : _closeButton };
-    ApplyVisualConstraints(buttonVisualConstraints, views);
+    _signinPromoView.closeButton.hidden = NO;
+    [_signinPromoView.closeButton addTarget:self
+                                     action:@selector(closeButtonAction:)
+                           forControlEvents:UIControlEventTouchUpInside];
 
     _signinPromoView.backgroundColor = [UIColor whiteColor];
     _signinPromoView.textLabel.text =
         l10n_util::GetNSString(IDS_IOS_SIGNIN_PROMO_BOOKMARKS);
   }
   return self;
 }
 
 - (void)layoutSubviews {
   // Adjust the text label preferredMaxLayoutWidth according self.frame.width,
   // so the text will adjust its height and not its width.
   CGFloat parentWidth = CGRectGetWidth(self.bounds);
   _signinPromoView.textLabel.preferredMaxLayoutWidth =
       parentWidth - 2 * _signinPromoView.horizontalPadding;
 
   // Re-layout with the new preferred width to allow the label to adjust its
   // height.
   [super layoutSubviews];
 }
 
 - (void)prepareForReuse {
   _closeButtonAction = nil;
   _signinPromoView.delegate = nil;
 }
 
 - (void)closeButtonAction:(id)sender {
-  if (_closeButtonAction) {
-    _closeButtonAction();
-  }
+  DCHECK(_closeButtonAction);
+  _closeButtonAction();
 }
 
 @end