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

mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl

 // Copyright 2013 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.
 
 {%- if variant -%}
 {%-   set variant_path = "%s-%s"|format(module.path, variant) -%}
 {%- else -%}
 {%-   set variant_path = module.path -%}
 {%- endif -%}
 
@@ skipping 34 matching lines @@
 #include "mojo/public/cpp/bindings/associated_interface_ptr.h"
 #include "mojo/public/cpp/bindings/associated_interface_ptr_info.h"
 #include "mojo/public/cpp/bindings/associated_interface_request.h"
 #include "mojo/public/cpp/bindings/clone_traits.h"
 #include "mojo/public/cpp/bindings/interface_ptr.h"
 #include "mojo/public/cpp/bindings/interface_request.h"
 #include "mojo/public/cpp/bindings/lib/equals_traits.h"
 #include "mojo/public/cpp/bindings/lib/control_message_handler.h"
 #include "mojo/public/cpp/bindings/lib/control_message_proxy.h"
 #include "mojo/public/cpp/bindings/lib/serialization.h"
-#include "mojo/public/cpp/bindings/lib/union_accessor.h"
 #include "mojo/public/cpp/bindings/native_struct.h"
 #include "mojo/public/cpp/bindings/raw_ptr_impl_ref_traits.h"
 #include "mojo/public/cpp/bindings/struct_ptr.h"
 #include "mojo/public/cpp/bindings/struct_traits.h"
 #include "mojo/public/cpp/bindings/thread_safe_interface_ptr.h"
 #include "mojo/public/cpp/bindings/union_traits.h"
 #include "{{module.path}}-shared.h"
 {%- for import in imports %}
 {%-   if variant %}
 #include "{{"%s-%s.h"|format(import.module.path, variant)}}"
@@ skipping 161 matching lines @@
 {#--- Union Serialization Helpers -#}
 {%  if unions %}
 {%-   for union in unions %}
 {%      include "union_traits_declaration.tmpl" %}
 {%-   endfor %}
 {%- endif %}
 
 }  // namespace mojo
 
 #endif  // {{header_guard}}