AX_ROLE_HORIZONTAL_RULE is not required.

ui/accessibility/platform/ax_platform_node_mac.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 "ui/accessibility/platform/ax_platform_node_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/strings/sys_string_conversions.h"
 #import "ui/accessibility/platform/ax_platform_node_delegate.h"
@@ skipping 43 matching lines @@
       {ui::AX_ROLE_EDITABLE_TEXT, NSAccessibilityTextFieldRole},
       {ui::AX_ROLE_FIGCAPTION, NSAccessibilityGroupRole},
       {ui::AX_ROLE_FIGURE, NSAccessibilityGroupRole},
       {ui::AX_ROLE_FOOTER, NSAccessibilityGroupRole},
       {ui::AX_ROLE_FORM, NSAccessibilityGroupRole},
       {ui::AX_ROLE_GRID, NSAccessibilityGridRole},
       {ui::AX_ROLE_GROUP, NSAccessibilityGroupRole},
       {ui::AX_ROLE_GROW_AREA, NSAccessibilityGrowAreaRole},
       {ui::AX_ROLE_HEADING, @"AXHeading"},
       {ui::AX_ROLE_HELP_TAG, NSAccessibilityHelpTagRole},
-      {ui::AX_ROLE_HORIZONTAL_RULE, NSAccessibilityGroupRole},
       {ui::AX_ROLE_IFRAME, NSAccessibilityGroupRole},
       {ui::AX_ROLE_IGNORED, NSAccessibilityUnknownRole},
       {ui::AX_ROLE_IMAGE, NSAccessibilityImageRole},
       {ui::AX_ROLE_IMAGE_MAP, NSAccessibilityGroupRole},
       {ui::AX_ROLE_IMAGE_MAP_LINK, NSAccessibilityLinkRole},
       {ui::AX_ROLE_INCREMENTOR, NSAccessibilityIncrementorRole},
       {ui::AX_ROLE_LABEL_TEXT, NSAccessibilityGroupRole},
       {ui::AX_ROLE_LINK, NSAccessibilityLinkRole},
       {ui::AX_ROLE_LIST, NSAccessibilityListRole},
       {ui::AX_ROLE_LIST_BOX, NSAccessibilityListRole},
@@ skipping 237 matching lines @@
   delete this;
 }
 
 gfx::NativeViewAccessible AXPlatformNodeMac::GetNativeViewAccessible() {
   if (!native_node_)
     native_node_.reset([[AXPlatformNodeCocoa alloc] initWithNode:this]);
   return native_node_.get();
 }
 
 }  // namespace ui