[mojo-bindings] Support reuse of native enum classes

mojo/public/tools/bindings/pylib/mojom/parse/ast.py

Index: mojo/public/tools/bindings/pylib/mojom/parse/ast.py
diff --git a/mojo/public/tools/bindings/pylib/mojom/parse/ast.py b/mojo/public/tools/bindings/pylib/mojom/parse/ast.py
index 6473b02b6ba292bd963d6dd9163c7db57b9348c3..9ef256ceaead3d12cad8cc5797f73d430ab41795 100644
--- a/mojo/public/tools/bindings/pylib/mojom/parse/ast.py
+++ b/mojo/public/tools/bindings/pylib/mojom/parse/ast.py
@@ -138,7 +138,7 @@ class Enum(Definition):
 
   def __init__(self, name, attribute_list, enum_value_list, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
-    assert isinstance(enum_value_list, EnumValueList)
+    assert isinstance(enum_value_list, EnumValueList) or enum_value_list is None
     super(Enum, self).__init__(name, **kwargs)
     self.attribute_list = attribute_list
     self.enum_value_list = enum_value_list