lock() {
    this.isLocked_ = true;
    this.forEachChild(function(chip) {
      chip.lock();
    });
    goog.dom.classlist.add(this.shadowInputElem_, constants.CssClass.INVISIBLE);
    var passphraseEncryptionLink =
        this.getElementByClass(constants.CssClass.PASSPHRASE_ENCRYPTION_LINK);
    goog.dom.classlist.add(
        passphraseEncryptionLink, constants.CssClass.INVISIBLE);
  }
Locks a ChipHolder, disallowing modifications to chips.