Add support for enums within structs and interfaces to mojom.

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

 class {{interface.name}}Proxy;
 class {{interface.name}}Stub;
 class {{interface.peer}};
 
 class {{interface.name}} {
  public:
   typedef {{interface.name}}Proxy _Proxy;
   typedef {{interface.name}}Stub _Stub;
   typedef {{interface.peer}} _Peer;
 
-{%- for method in interface.methods %}
+{#--- 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}}(
 {%-   for param in method.parameters -%}
     {{param.kind|cpp_const_wrapper_type}} {{param.name}}
 {%-     if not loop.last %}, {% endif -%}
 {%-   endfor -%}
   ) = 0;
 {%- endfor %}
 };