Mojo C++ bindings: add test cases for union custom typemapping.

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

 // Copyright 2016 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;
 
 // TODO(yzshen): Rename *WithTraits* types to something more readable.
 
 struct NestedStructWithTraits {
   int32 value;
@@ skipping 35 matching lines @@
 // The custom type for MoveOnlyStructWithTraits is not clonable. Test that
 // this container can compile as long as Clone() is not used.
 struct MoveOnlyStructWithTraitsContainer {
   MoveOnlyStructWithTraits f_struct;
 };
 
 struct StructWithTraitsForUniquePtr {
   int32 f_int32;
 };
 
+union UnionWithTraits {
+  int32 f_int32;
+  NestedStructWithTraits f_struct;
+};
+
 interface TraitsTestService {
   EchoStructWithTraits(StructWithTraits s) => (StructWithTraits passed);
 
   EchoTrivialStructWithTraits(TrivialStructWithTraits s) =>
       (TrivialStructWithTraits passed);
 
   EchoMoveOnlyStructWithTraits(MoveOnlyStructWithTraits s) =>
       (MoveOnlyStructWithTraits passed);
 
   EchoNullableMoveOnlyStructWithTraits(MoveOnlyStructWithTraits? s) =>
       (MoveOnlyStructWithTraits? passed);
 
   EchoEnumWithTraits(EnumWithTraits e) => (EnumWithTraits passed);
 
   EchoStructWithTraitsForUniquePtr(StructWithTraitsForUniquePtr e) => (
       StructWithTraitsForUniquePtr passed);
 
   EchoNullableStructWithTraitsForUniquePtr(StructWithTraitsForUniquePtr? e) => (
       StructWithTraitsForUniquePtr? passed);
+
+  EchoUnionWithTraits(UnionWithTraits u) => (UnionWithTraits passed);

[Fady Samuel, 2016/08/08 23:02:46]

optional nit: I've found tests are a lot more readable if this is [Sync].

[yzshen1, 2016/08/08 23:07:38]

Yeah, that is probably true.

If you don't mind, I would like to leave this unchanged to be more consistent
with the other test cases in the same file.

WDYT?
Thanks!

[Fady Samuel, 2016/08/08 23:09:03]

Yea, of course, no problem. lgtm

 };