Support default values for POD struct fields in mojom IDL.

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(field) -%}
+{%-   if field.kind|is_string_kind -%}
+set_{{field.name}}(mojo::String({{field.default}}));
+{%-   elif field.kind|is_object_kind -%}
+{
+  {{field.kind|cpp_wrapper_type}}::Builder tmp;
+  // TODO
+  set_{{field.name}}(tmp.Finish());
+}
+{%-   else -%}
+set_{{field.name}}({{field.default}});
+{%-   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 %}
+  {{set_default(pf.field)|indent(2)}}

[darin (slow to review), 2014/01/21 23:47:20]

Perhaps we should defer setting default values until Finish() is called? That
way we don't waste memory writing default values for fields that the user sets?

Hmm, ah... but we don't keep track of what methods are called, so we can't know
if, for example, a null string field was intentional or not.

OK, nevermind.

+{%- endfor %}
 }
 
 {{class_name}} {{class_name}}::Builder::Finish() {
   Data* data = NULL;
   std::swap(data, data_);
   return mojo::internal::Wrap(data);
 }