Introduce DEFINE_AX_OBJECT_TYPE_CASTS to replace manual toFoo() in accessibility child class

Source/core/accessibility/AXSVGRoot.h

Index: Source/core/accessibility/AXSVGRoot.h
diff --git a/Source/core/accessibility/AXSVGRoot.h b/Source/core/accessibility/AXSVGRoot.h
index 46a76860d33be86fe956c0c7145b292b63e8bad1..4febd09f851b7c24f95ace10a070944a6a04b44f 100644
--- a/Source/core/accessibility/AXSVGRoot.h
+++ b/Source/core/accessibility/AXSVGRoot.h
@@ -50,11 +50,7 @@ private:
     virtual bool isAXSVGRoot() const OVERRIDE { return true; }
 };
 
-inline AXSVGRoot* toAXSVGRoot(AXObject* object)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isAXSVGRoot());
-    return static_cast<AXSVGRoot*>(object);
-}
+DEFINE_AX_OBJECT_TYPE_CASTS(AXSVGRoot, isAXSVGRoot());
 
 } // namespace WebCore