Implement unions as members of structs.

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

Index: mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
index 03120024fda3dda88c0df3f02b8b85892323b5a4..503e19decb942e889348a60bc0f6ea18a2eb4bb3 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
@@ -63,6 +63,12 @@ typedef mojo::StructPtr<{{union.name}}> {{union.name}}Ptr;
 {%    endif %}
 {%- endfor %}
 
+{#--- Unions must be declared first because they can be members of structs #}
+{#--- Unions #}
+{%  for union in unions %}
+{%    include "wrapper_union_class_declaration.tmpl" %}
+{%- endfor %}
+
 {#--- NOTE: Non-inlined structs may have pointers to inlined structs, so we  #}
 {#---       need to fully define inlined structs ahead of the others.        #}
 
@@ -80,11 +86,6 @@ typedef mojo::StructPtr<{{union.name}}> {{union.name}}Ptr;
 {%    endif %}
 {%- endfor %}
 
-{#--- Unions #}
-{%  for union in unions %}
-{%    include "wrapper_union_class_declaration.tmpl" %}
-{%- endfor %}
-
 {#--- Interfaces -#}
 {%  for interface in interfaces %}
 {%    include "interface_declaration.tmpl" %}
@@ -110,18 +111,17 @@ typedef mojo::StructPtr<{{union.name}}> {{union.name}}Ptr;
 {%    include "interface_response_validator_declaration.tmpl" %}
 {%- endfor %}
 
-{%- import "serialization_macros.tmpl" as serialization_macros %}
 {#--- Struct Serialization Helpers -#}
 {%  if structs %}
 {%    for struct in structs %}
-{{      serialization_macros.declare_serialization(struct.name) }}
+{%      include "struct_serialization_declaration.tmpl" %}
 {%-   endfor %}
 {%- endif %}
 
 {#--- Union Serialization Helpers -#}
 {%  if unions %}
 {%    for union in unions %}
-{{      serialization_macros.declare_serialization(union.name) }}
+{%      include "union_serialization_declaration.tmpl" %}
 {%-   endfor %}
 {%- endif %}