Add support for enums within structs and interfaces to mojom.

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

 class {{struct.name}} {
  public:
   typedef internal::{{struct.name}}_Data Data;
+{%- for enum in struct.enums %}
+  typedef internal::{{enum.name}} {{enum.name}};
+{%- endfor %}
 
   {{struct.name}}() : data_(NULL) {
   }
 
   template <typename U>
   {{struct.name}}(const U& u, mojo::Buffer* buf = mojo::Buffer::current()) {
     *this = mojo::TypeConverter<{{struct.name}},U>::ConvertFrom(u, buf);
   }
 
   template <typename U>
@@ skipping 57 matching lines @@
   };
 
  private:
   friend class mojo::internal::WrapperHelper<{{struct.name}}>;
 
   struct Wrap {};
   {{struct.name}}(Wrap, const Data* data) : data_(data) {}
 
   const Data* data_;
 };