Fire AX notifications on list box options.

Source/modules/accessibility/AXListBox.h

Index: Source/modules/accessibility/AXListBox.h
diff --git a/Source/modules/accessibility/AXListBox.h b/Source/modules/accessibility/AXListBox.h
index ce3c82e41274f6ad7d7a25a5245b3620307120f2..e625796a0b4c6525ad24c496c03142c76779d35a 100644
--- a/Source/modules/accessibility/AXListBox.h
+++ b/Source/modules/accessibility/AXListBox.h
@@ -45,6 +45,14 @@ public:
     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