Fire AX notifications on list box options.

Source/modules/accessibility/AXListBox.h

 /*
  * Copyright (C) 2008 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 27 matching lines @@
 class AXListBox final : public AXLayoutObject {
 
 private:
     AXListBox(LayoutObject*, AXObjectCacheImpl*);
 
 public:
     static PassRefPtr<AXListBox> create(LayoutObject*, AXObjectCacheImpl*);
     virtual ~AXListBox();
 
     virtual AccessibilityRole roleValue() const override;
+    virtual bool isAXListBox() const override { return true; }
+    virtual bool shouldFocusActiveDescendant() const override { return true; }
+    virtual AXObject* activeDescendant() const override;
+
+    void activeIndexChanged();
+
+private:
+    int m_activeIndex;
 };
 
 } // namespace blink
 
 #endif // AXListBox_h