Mojo: Add support for constants to the IDL compiler.

mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 
 {%- macro pass_params(parameters) %}
 {%-   for param in parameters %}
 {%-     if param.kind|is_object_kind -%}
 mojo::internal::Wrap(params->{{param.name}}())
 {%-     elif param.kind|is_handle_kind -%}
 mojo::MakePassable(params->{{param.name}}()).Pass()
@@ skipping 31 matching lines @@
   params->set_{{param.name}}(in_{{param.name}}.release());
 {%-     else %}
   params->set_{{param.name}}(in_{{param.name}});
 {%-     endif %}
 {%-   endfor %}
   mojo::Message message;
   params->EncodePointersAndHandles(message.mutable_handles());
   builder.Finish(&message);
 {%- endmacro %}
 
+{#--- Begin #}
+
+{#--- Constants #}
+{%  for constant in interface.constants %}
+const {{constant.kind|cpp_pod_type}} {{interface.name}}::{{constant.name}} = {{constant.value|expression_to_text(module)}};
+{%- endfor %}
+
 {#--- ForwardToCallback definition #}
 {%- for method in interface.methods -%}
 {%-   if method.response_parameters != None %}
 class {{class_name}}_{{method.name}}_ForwardToCallback
     : public mojo::MessageReceiver {
  public:
   {{class_name}}_{{method.name}}_ForwardToCallback(
       const {{interface_macros.declare_callback(method)}}& callback)
       : callback_(callback) {
   }
@@ skipping 157 matching lines @@
       return true;
 {%-     else %}
       break;
 {%-     endif %}
     }
 {%-   endfor %}
   }
 {%- endif %}
   return false;
 }