Add support for enums within structs and interfaces to mojom.

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set class_name = struct.name ~ "_Data" -%}
 class {{class_name}} {
  public:
   typedef {{struct.name}} Wrapper;
 
+{#--- Enums #}
+{%- for enum in struct.enums %}
+{%    macro enum_def() %}{% include "enum_declaration.tmpl" %}{% endmacro %}
+  {{enum_def()|indent(2)}}
+{%- endfor %}
+
   static {{class_name}}* New(mojo::Buffer* buf, mojo::Buffer::Destructor dtor = NULL);
 
 {#--- Setters -#}
 {{ struct_macros.setters(struct) }}
 
 {#--- Getters -#}
 {{ struct_macros.getters(struct) }}
 
  private:
   friend class mojo::internal::ObjectTraits<{{class_name}}>;
 
   {{class_name}}();
   ~{{class_name}}();  // NOT IMPLEMENTED
 
   mojo::internal::StructHeader _header_;
 {{ struct_macros.fields(struct) }}
 };
 MOJO_COMPILE_ASSERT(sizeof({{class_name}}) == {{struct.packed|struct_size}},
                     bad_sizeof_{{class_name}});