Implement python mojo bindings unions.

mojo/public/interfaces/bindings/tests/test_unions.mojom

 // Copyright 2014 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.
 
 module mojo.test;
 
 enum AnEnum {
   FIRST, SECOND
 };
 
@@ skipping 51 matching lines @@
   map<string, PodUnion>? pod_union_map;
   map<string, PodUnion?>? nullable_pod_union_map;
 };
 
 struct SmallStructNonNullableUnion {
   PodUnion pod_union;
 };
 
 struct SmallObjStruct {
   ObjectUnion obj_union;
+  int8 f_int8;
 };
 
 interface SmallCache {
   SetIntValue(int64 int_value);
   GetIntValue() => (int64 int_value);
 };
 
 interface UnionInterface {
   Echo(PodUnion in_val) => (PodUnion out_val);
 };
 
 struct TryNonNullStruct {
   DummyStruct? nullable;
   DummyStruct non_nullable;
 };