Implement unions as members of structs.

mojo/public/tools/bindings/generators/cpp_templates/module-internal.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.
 
 {%- set header_guard = "%s_INTERNAL_H_"|
         format(module.path|upper|replace("/","_")|replace(".","_")) %}
 
 #ifndef {{header_guard}}
 #define {{header_guard}}
 
@@ skipping 32 matching lines @@
 {%  for struct in structs %}
 class {{struct.name}}_Data;
 {%- endfor %}
 
 {%  for union in unions %}
 class {{union.name}}_Data;
 {%- endfor %}
 
 #pragma pack(push, 1)
 
+{#--- Unions must be declared first because they can be members of structs #}
+{#--- Union class declarations #}
+{%  for union in unions %}
+{%    include "union_declaration.tmpl" %}
+{%- endfor %}
+
 {#--- Class declarations #}
 {%  for struct in structs %}
 {%    include "struct_declaration.tmpl" %}
 {%- endfor %}
 
-{%  for union in unions %}
-{%    include "union_declaration.tmpl" %}
-{%- endfor %}
-
 #pragma pack(pop)
 
 }  // namespace internal
 {%- for namespace in namespaces_as_array|reverse %}
 }  // namespace {{namespace}}
 {%- endfor %}
 
 #endif  // {{header_guard}}