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

mojo/public/cpp/bindings/tests/struct_with_traits_impl.h

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 89 matching lines @@
 // A type which knows how to look like a mojo::test::MoveOnlyStructWithTraits
 // mojom type by way of mojo::StructTraits.
 class MoveOnlyStructWithTraitsImpl {
  public:
   MoveOnlyStructWithTraitsImpl();
   MoveOnlyStructWithTraitsImpl(MoveOnlyStructWithTraitsImpl&& other);
   ~MoveOnlyStructWithTraitsImpl();
 
   ScopedHandle& get_mutable_handle() { return handle_; }
 
+  MoveOnlyStructWithTraitsImpl& operator=(MoveOnlyStructWithTraitsImpl&& other);
+
  private:
   ScopedHandle handle_;
   DISALLOW_COPY_AND_ASSIGN(MoveOnlyStructWithTraitsImpl);
 };
 
 }  // namespace test
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_H_