Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/bindings/scripts/v8_types.py

Index: Source/bindings/scripts/v8_types.py
diff --git a/Source/bindings/scripts/v8_types.py b/Source/bindings/scripts/v8_types.py
index 5cce99914b0f17c5b10af91acb4bc664e86439f0..746dde3da43e1e3b3d409439c2c1331c0f97dc3c 100644
--- a/Source/bindings/scripts/v8_types.py
+++ b/Source/bindings/scripts/v8_types.py
@@ -138,15 +138,13 @@ def cpp_type(idl_type, extended_attributes=None, raw_type=False, used_as_argumen
             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':
+        if idl_type.is_nullable or extended_attributes.get('TreatNullAs') == 'NullString':
+            if extended_attributes.get('TreatUndefinedAs') == 'NullString':
+                return 'WithUndefinedOrNullCheck'
             return 'WithNullCheck'
-        return 'WithUndefinedOrNullCheck'
+        return ''
 
     extended_attributes = extended_attributes or {}
     idl_type = idl_type.preprocessed_type