Make the fuchsia mojo/public repo the source of truth.

mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_declaration.tmpl

-class {{union.name}} {
- public:
-  using Data_ = internal::{{union.name}}_Data;
-  using Tag = Data_::{{union.name}}_Tag;
-
-  static {{union.name}}Ptr New();
-
-  template <typename U>
-  static {{union.name}}Ptr From(const U& u) {
-    return mojo::TypeConverter<{{union.name}}Ptr, U>::Convert(u);
-  }
-
-  template <typename U>
-  U To() const {
-    return mojo::TypeConverter<U, {{union.name}}>::Convert(*this);
-  }
-
-  {{union.name}}();
-  ~{{union.name}}();
-
-{%  if union|is_cloneable_kind %}
-  {{union.name}}Ptr Clone() const;
-{%- endif %}
-  bool Equals(const {{union.name}}& other) const;
-
-  Tag which() const {
-    return tag_;
-  }
-
-{%  for field in union.fields %}
-  bool is_{{field.name}}() const;
-  {{field.kind|cpp_union_getter_return_type}} get_{{field.name}}() const;
-  void set_{{field.name}}({{field.kind|cpp_const_wrapper_type}} {{field.name}});
-{%- endfor %}
-  bool has_unknown_tag() const;
-
- private:
-  friend class mojo::internal::UnionAccessor<{{union.name}}>;
-  union Union_ {
-    Union_() {}
-    ~Union_() {}
-{%  for field in union.fields %}
-{%    if field.kind|is_object_kind or field.kind|is_any_handle_kind
-          or field.kind|is_interface_kind -%}
-    {{field.kind|cpp_wrapper_type}}* {{field.name}};
-{%    else -%}
-    {{field.kind|cpp_wrapper_type}} {{field.name}};
-{%    endif -%}
-{%- endfor %}
-  };
-  void SwitchActive(Tag new_active);
-  void SetActive(Tag new_active);
-  void DestroyActive();
-  Tag tag_;
-  Union_ data_;
-};