Mojo C++ bindings: support mapping T and T? differently.

mojo/public/cpp/bindings/tests/struct_with_traits_impl.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.h"
 
 namespace mojo {
 namespace test {
 
 NestedStructWithTraitsImpl::NestedStructWithTraitsImpl() {}
 NestedStructWithTraitsImpl::NestedStructWithTraitsImpl(int32_t in_value)
     : value(in_value) {}
 
 StructWithTraitsImpl::StructWithTraitsImpl() {}
 
 StructWithTraitsImpl::~StructWithTraitsImpl() {}
 
 StructWithTraitsImpl::StructWithTraitsImpl(const StructWithTraitsImpl& other) =
     default;
 
 MoveOnlyStructWithTraitsImpl::MoveOnlyStructWithTraitsImpl() {}
 
 MoveOnlyStructWithTraitsImpl::MoveOnlyStructWithTraitsImpl(
     MoveOnlyStructWithTraitsImpl&& other) = default;
 
 MoveOnlyStructWithTraitsImpl::~MoveOnlyStructWithTraitsImpl() {}
 
+MoveOnlyStructWithTraitsImpl& MoveOnlyStructWithTraitsImpl::operator=(
+    MoveOnlyStructWithTraitsImpl&& other) = default;
+
 }  // namespace test
 }  // namespace mojo