Mojo: Add support for constants to the IDL compiler.

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

 {%- for method in interface.methods %}
   var k{{interface.name}}_{{method.name}}_Name = {{method.ordinal}};
 {%- endfor %}
 
   function {{interface.name}}Proxy(receiver) {
     this.receiver_ = receiver;
   }
 
 {%- for method in interface.methods %}
   {{interface.name}}Proxy.prototype.{{method.name|stylize_method}} = function(
@@ skipping 91 matching lines @@
       return Promise.reject(Error("Unhandled message: " + reader.messageName));
     }
   };
 
 {#--- Enums #}
 {%  from "enum_definition.tmpl" import enum_def -%}
 {%  for enum in interface.enums %}
   {{enum_def("%sProxy.%s"|format(interface.name, enum.name), enum, module)}}
   {{interface.name}}Stub.{{enum.name}} = {{interface.name}}Proxy.{{enum.name}};
 {%-  endfor %}
+
+{#--- Constants. #}
+{%  for constant in interface.constants %}
+  {{interface.name}}Proxy.{{constant.name}} = {{constant.value|expression_to_text(module)}};
+  {{interface.name}}Stub.{{constant.name}} = {{interface.name}}Proxy.{{constant.name}};
+{%  endfor %}