Implement unions as members of structs.

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

-{%- macro declare_serialization(name) %}
+{%- macro declare_struct_serialization(name) %}
 size_t GetSerializedSize_(const {{name}}Ptr& input);
 void Serialize_({{name}}Ptr input, mojo::internal::Buffer* buffer,
                 internal::{{name}}_Data** output);
 void Deserialize_(internal::{{name}}_Data* input,
                   {{name}}Ptr* output);
 {%- endmacro %}
+
+{%- macro declare_union_serialization(name) %}
+size_t GetSerializedSize_(const {{name}}Ptr& input);
+size_t GetUnionInUnionSerializedSize_(const {{name}}Ptr& input);
+size_t GetStandaloneSerializeSize_(const {{name}}Ptr& input);
+void SerializeUnion_({{name}}Ptr input, mojo::internal::Buffer* buffer,
+                     internal::{{name}}_Data** output, bool inlined);
+void Deserialize_(internal::{{name}}_Data* input,
+                  {{name}}Ptr* output);
+{%- endmacro %}