Implement unions as members of structs.

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;
 
 union PodUnion {
   int8   f_int8;
   int8   f_int8_other;
   uint8  f_uint8;
   int16  f_int16;
   uint16 f_uint16;
   int32  f_int32;
   uint32 f_uint32;
   int64  f_int64;
   uint64 f_uint64;
   float  f_float;
   double f_double;
   bool   f_bool;
 };
 
 union ObjectUnion {
   int8   f_int8;
   string f_string;
 };
+
+struct DummyStruct {
+  int8 f_int8;
+};
+
+struct SmallStruct {
+  DummyStruct? dummy_struct;
+  PodUnion? pod_union;
+  array<PodUnion>? pod_union_array;
+  array<DummyStruct>? s_array;
+};
+
+struct SmallStructNonNullableUnion {
+  PodUnion pod_union;
+};
+
+struct SmallObjStruct {
+  ObjectUnion obj_union;
+};