Upstream the grammer for union types to idl_parser

tools/idl_parser/test_parser/interface_web.idl

Index: tools/idl_parser/test_parser/interface_web.idl
diff --git a/tools/idl_parser/test_parser/interface_web.idl b/tools/idl_parser/test_parser/interface_web.idl
index a2c9ae6346609a1e54ed0db50108489bc9eab497..007ac7ffed8dc8447b476ca2f422c014b8241554 100644
--- a/tools/idl_parser/test_parser/interface_web.idl
+++ b/tools/idl_parser/test_parser/interface_web.idl
@@ -381,3 +381,17 @@ interface MyIfaceSerializer {
 interface MyIfaceFrozenArray {
   readonly attribute FrozenArray<DOMString> foo;
 };
+
+/* TREE
+ *Interface(MyIfaceUnion)
+ *  Attribute(foo)
+ *    Type()
+ *      UnionType()
+ *        Type()
+ *          PrimitiveType(DOMString)
+ *        Type()
+ *          PrimitiveType(long)
+ */
+interface MyIfaceUnion {
+  attribute (DOMString or long) foo;
+};