Upstream the grammer for union types to idl_parser

tools/idl_parser/test_parser/interface_web.idl

 /* Copyright (c) 2013 The Chromium Authors. All rights reserved.
    Use of this source code is governed by a BSD-style license that can be
   found in the LICENSE file. */
 
 /* Test Interface productions
 
 Run with --test to generate an AST and verify that all comments accurately
 reflect the state of the Nodes.
 
 BUILD Type(Name)
@@ skipping 363 matching lines @@
  *Interface(MyIfaceFrozenArray)
  *  Attribute(foo)
  *    Type()
  *      FrozenArray()
  *        Type()
  *          PrimitiveType(DOMString)
  */
 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;
+};