IDL: Use Nullable for union type return value

Source/bindings/scripts/v8_interface.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 # coding=utf-8
 #
 # 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 916 matching lines @@
 
 
 ################################################################################
 # Special operations (methods)
 # http://heycam.github.io/webidl/#idl-special-operations
 ################################################################################
 
 def property_getter(getter, cpp_arguments):
     def is_null_expression(idl_type):
         if idl_type.is_union_type:
-            return ' && '.join('!result%sEnabled' % i
+            return ' && '.join('result%s.isNull()' % i
                                for i, _ in enumerate(idl_type.member_types))
         if idl_type.name == 'String':
             return 'result.isNull()'
         if idl_type.is_interface_type:
             return '!result'
         return ''
 
     idl_type = getter.idl_type
     extended_attributes = getter.extended_attributes
     is_raises_exception = 'RaisesException' in extended_attributes
@@ skipping 160 matching lines @@
         deleter = next(
             method
             for method in interface.operations
             if ('deleter' in method.specials and
                 len(method.arguments) == 1 and
                 str(method.arguments[0].idl_type) == 'DOMString'))
     except StopIteration:
         return None
 
     return property_deleter(deleter)