Return a string with the definition of this field.

def Definition(self):
    """Return a string with the definition of this field."""
    result = self._FormatDescriptionComment()

    result += "  enum %s {\n" % self.enum_name
    for k, v in sorted(iteritems(self.reverse_enum)):
      result += "    %s = %s;\n" % (v, k)

    result += "  }\n"

    result += self._FormatField()
    return result