public static ProgressToken fromSerializedString(String serializedString) {
        byte[] serialized;
        try {
            serialized = Base64.getUrlDecoder().decode(serializedString);
        } catch (IllegalArgumentException e) {
            // Legacy visitor tokens were encoded with MIME Base64 which may fail decoding as URL-safe.
            // Try again with MIME decoder to avoid breaking upgrade scenarios.
            // TODO(vekterli): remove once this is no longer a risk.
            serialized = Base64.getMimeDecoder().decode(serializedString);
        }
        return new ProgressToken(serialized);
    }
Returns a string (base64) encoding of the serial form of this token
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java