IDL: Use V8StringResource<WithNullCheck> for nullable DOMString

Source/bindings/scripts/v8_types.py

 # Copyright (C) 2013 Google 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:
 #
 #     * 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
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 119 matching lines @@
         raw_type:
             bool, True if idl_type's raw/primitive C++ type should be returned.
         used_as_argument:
             bool, True if the C++ type is used as an argument of a method.
         used_as_variadic_argument:
             bool, True if the C++ type is used as a variadic argument of a method.
         used_in_cpp_sequence:
             bool, True if the C++ type is used as an element of an array or sequence.
     """
     def string_mode():
+        if idl_type.is_nullable:
+            return 'WithNullCheck'
         # FIXME: the Web IDL spec requires 'EmptyString', not 'NullString',
         # but we use NullString for performance.
         if extended_attributes.get('TreatNullAs') != 'NullString':
             return ''
         if extended_attributes.get('TreatUndefinedAs') != 'NullString':
             return 'WithNullCheck'
         return 'WithUndefinedOrNullCheck'
 
     extended_attributes = extended_attributes or {}
     idl_type = idl_type.preprocessed_type
@@ skipping 548 matching lines @@
 
 def literal_cpp_value(idl_type, idl_literal):
     """Converts an expression that is a valid C++ literal for this type."""
     # FIXME: add validation that idl_type and idl_literal are compatible
     literal_value = str(idl_literal)
     if idl_type.base_type in CPP_UNSIGNED_TYPES:
         return literal_value + 'u'
     return literal_value
 
 IdlType.literal_cpp_value = literal_cpp_value