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..d367cd1c8f2a80397b039816dbb5bfbb016dadae 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" %}