Mojo: Align string sizes when serializing a native type.

mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.cc

 // 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.
 
 #include "mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h"
 #include "mojo/public/interfaces/bindings/tests/struct_with_traits.mojom.h"
 
 namespace mojo {
 
 // static
 bool StructTraits<test::StructWithTraits, test::StructWithTraitsImpl>::Read(
     test::StructWithTraits_Reader r,
     test::StructWithTraitsImpl* out) {
   out->set_bool(r.f_bool());
   out->set_uint32(r.f_uint32());
   out->set_uint64(r.f_uint64());
   out->set_string(r.f_string().as_string());
-  return true;
+  return r.f_string() == r.f_string2();
 }
 
 }  // namespace mojo