Add MarkRole in blink side to expose mark tag properly

public/web/WebAXEnums.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 112 matching lines @@
     WebAXRoleLabel,
     WebAXRoleLegend,
     WebAXRoleLink,
     WebAXRoleListBoxOption,
     WebAXRoleListBox,
     WebAXRoleListItem,
     WebAXRoleListMarker,
     WebAXRoleList,
     WebAXRoleLog,
     WebAXRoleMain,
+    WebAXRoleMark,
     WebAXRoleMarquee,
     WebAXRoleMath,
     WebAXRoleMenuBar,
     WebAXRoleMenuButton,
     WebAXRoleMenuItem,
     WebAXRoleMenuItemCheckBox,
     WebAXRoleMenuItemRadio,
     WebAXRoleMenuListOption,
     WebAXRoleMenuListPopup,
     WebAXRoleMenu,
@@ skipping 152 matching lines @@
 // because on some platforms this determines how the accessible description
 // is exposed.
 enum WebAXDescriptionFrom {
     WebAXDescriptionFromPlaceholder,
     WebAXDescriptionFromRelatedElement
 };
 
 } // namespace blink
 
 #endif