Add field-initializing factory static methods to generated mojo unions.

mojo/public/cpp/bindings/lib/union_accessor.h

-// Copyright 2015 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_LIB_UNION_ACCESSOR_H_
-#define MOJO_PUBLIC_CPP_BINDINGS_LIB_UNION_ACCESSOR_H_
-
-namespace mojo {
-namespace internal {
-
-// When serializing and deserializing Unions, it is necessary to access
-// the private fields and methods of the Union. This allows us to do that
-// without leaking those same fields and methods in the Union interface.
-// All Union wrappers are friends of this class allowing such access.
-template <typename U>
-class UnionAccessor {
- public:
-  explicit UnionAccessor(U* u) : u_(u) {}
-
-  typename U::Union_* data() { return &(u_->data_); }
-
-  typename U::Tag* tag() { return &(u_->tag_); }
-
-  void SwitchActive(typename U::Tag new_tag) { u_->SwitchActive(new_tag); }
-
- private:
-  U* u_;
-};
-
-}  // namespace internal
-}  // namespace mojo
-
-#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_UNION_ACCESSOR_H_