Mojo: Add support for constants to the IDL compiler.

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
 class {{interface.name}}Stub;
 {% if interface.peer %}
 class {{interface.peer}};
 {% endif %}
 
 class {{interface.name}} {
  public:
   typedef {{interface.name}}Proxy _Proxy;
   typedef {{interface.name}}Stub _Stub;
 {% if interface.peer %}
   typedef {{interface.peer}} _Peer;
 {% else %}
   typedef mojo::NoInterface _Peer;
 {% endif %}
 
+{#--- Constants #}
+{%- for constant in interface.constants %}
+  static const {{constant.kind|cpp_pod_type}} {{constant.name}};
+{%- endfor %}
+
 {#--- Enums #}
 {%- for enum in interface.enums %}
 {%    macro enum_def() %}{% include "enum_declaration.tmpl" %}{% endmacro %}
   {{enum_def()|indent(2)}}
 {%- endfor %}
 
 {#--- Methods #}
 {%- for method in interface.methods %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };