[ObjC ARC] Converts ios/chrome/browser/ui/stack_view:stack_view to ARC.

ios/chrome/browser/ui/stack_view/close_button.mm

 // Copyright 2014 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/stack_view/close_button.h"
 
 #include "base/logging.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation CloseButton {
-  id _accessibilityTarget;  // weak
+  __weak id _accessibilityTarget;
   SEL _accessibilityAction;
 }
 
 - (void)addAccessibilityElementFocusedTarget:(id)accessibilityTarget
                                       action:(SEL)accessibilityAction {
   DCHECK(!accessibilityTarget ||
          [accessibilityTarget respondsToSelector:accessibilityAction]);
   _accessibilityTarget = accessibilityTarget;
   _accessibilityAction = accessibilityAction;
 }
 
 - (void)accessibilityElementDidBecomeFocused {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
   [_accessibilityTarget performSelector:_accessibilityAction withObject:self];
+#pragma clang diagnostic pop
 }
 
 @end