IDL: Add support for [TreatNullAs=EmptyString]

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 8508ad81728eec92599480f46aeca2e2609fc9fa..efa460c8906b36f02a24e3d99b73a3714a195eb6 100644
--- a/Source/bindings/scripts/v8_types.py
+++ b/Source/bindings/scripts/v8_types.py
@@ -138,12 +138,12 @@ 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():
-        # FIXME: the Web IDL spec requires 'EmptyString', not 'NullString',
-        # but we use NullString for performance.
+        if extended_attributes.get('TreatNullAs') == 'EmptyString':
+            return 'TreatNullAsEmptyString'
         if idl_type.is_nullable or extended_attributes.get('TreatNullAs') == 'NullString':
             if extended_attributes.get('TreatUndefinedAs') == 'NullString':
-                return 'WithUndefinedOrNullCheck'
-            return 'WithNullCheck'
+                return 'TreatNullAndUndefinedAsNullString'
+            return 'TreatNullAsNullString'
         return ''
 
     extended_attributes = extended_attributes or {}