Support html br element in AX Tree.

Source/modules/accessibility/AXObject.h

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 149 matching lines @@
     TextFieldRole,
     TimeRole,
     TimerRole,
     ToggleButtonRole,
     ToolbarRole,
     TreeGridRole,
     TreeItemRole,
     TreeRole,
     UserInterfaceTooltipRole,
     WebAreaRole,
+    LineBreakRole,
     WindowRole,
     NumRoles
 };
 
 enum AccessibilityTextSource {
     AlternativeText,
     ChildrenText,
     SummaryText,
     HelpText,
     VisibleText,
@@ skipping 451 matching lines @@
     // functions called here may only search up the tree (ancestors), not down.
     void updateCachedAttributeValuesIfNeeded() const;
 };
 
 #define DEFINE_AX_OBJECT_TYPE_CASTS(thisType, predicate) \
     DEFINE_TYPE_CASTS(thisType, AXObject, object, object->predicate, object.predicate)
 
 } // namespace blink
 
 #endif // AXObject_h