Fix a bug with mojom imports where 2 imports with the same namespace would

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|substitute_namespace(imports) ~ ")")}}
 {#--- 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 33 matching lines @@
 {%-     endcall %}
 {%-   endfilter %}
 {%- endfor %}
 }
 
 {{class_name}} {{class_name}}::Builder::Finish() {
   Data* data = NULL;
   std::swap(data, data_);
   return mojo::internal::Wrap(data);
 }