Fix bug with using enums as default values in mojom. We were previously

mojo/public/bindings/generators/cpp_templates/struct_builder_definition.tmpl

 {%- set class_name = struct.name -%}
 {%- set dtor_name = [] %}
 {%- if struct|is_struct_with_handles %}
 {%-   set _ = dtor_name.append("&internal::%s_Data_Destructor"|format(struct.name)) %}
 {%- else %}
 {%-   set _ = dtor_name.append("NULL") %}
 {%- endif %}
 
 {%- macro set_default(kind, value, depth) -%}
 {#--- Strings ---#}
 {%-   if kind|is_string_kind -%}
-{{caller("mojo::String(" ~ value ~ ")")}}
+{{caller("mojo::String(" ~ value|expression_to_text(module) ~ ")")}}
 {#--- Arrays ---#}
 {%-   elif kind|is_array_kind %}
 {%-     set _ = value|verify_token_type("ARRAY") %}
 {
   {{kind|cpp_wrapper_type}}::Builder tmp{{depth}}({{value[1]|length}});
 {%-     for element in value[1] %}
 {%-       filter indent(2) %}
 {%-         call(result) set_default(kind.kind, element, depth+1) %}
 tmp{{depth}}[{{loop.index0}}] = {{result}};
 {%-         endcall %}
@@ skipping 13 matching lines @@
 {%-     filter indent(2) %}
 {%-       call(result) set_default(subfield.kind, element, depth+1) %}
 tmp{{depth}}.set_{{subfield.name}}({{result}});
 {%-       endcall %}
 {%-     endfilter %}
 {%-   endfor -%}
   {{caller("tmp" ~ depth ~ ".Finish()")|indent(2)}}
 }
 {#--- POD types ---#}
 {%-   else -%}
-{{caller(value|substitute_namespace(imports))}}
+{{caller(value|expression_to_text(module))}}
 {%-   endif %}
 {%- endmacro %}
 
 {{class_name}}::Builder::Builder(mojo::Buffer* buf)
     : data_(Data::New(buf, {{dtor_name[0]}})) {
 {%- for pf in struct.packed.packed_fields if pf.field.default %}
 {%-   filter indent(2) %}
 {%-     call(result) set_default(pf.field.kind, pf.field.default, 0) %}
 set_{{pf.field.name}}({{result}});
 {%-     endcall %}
 {%-   endfilter %}
 {%- endfor %}
 }
 
 {{class_name}} {{class_name}}::Builder::Finish() {
   Data* data = NULL;
   std::swap(data, data_);
   return mojo::internal::Wrap(data);
 }