Implement python mojo bindings unions.

mojo/public/tools/bindings/generators/python_templates/module_macros.tmpl

 # Copyright 2014 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.
 
 {%- macro enum_values(enum) -%}
 [
 {% for field in enum.fields %}
   ('{{field.name}}', {{field.resolved_value}}),
 {% endfor %}
 ]
@@ skipping 19 matching lines @@
   'fields': [
 {%  for byte in struct.bytes %}
 {%    if byte.packed_fields %}
     {{byte|field_group}},
 {%    endif %}
 {% endfor %}
   ],
 {% endif %}
 }
 {%- endmacro -%}
+
+{%- macro union_descriptor(union) -%}
+{
+  'fields': [
+{% for field in union.fields %}
+    {{field|union_field_descriptor}},
+{% endfor %}
+  ],
+ }
+{%- endmacro -%}