getTitle_(action) {
    switch (action) {
      case ext.constants.Actions.USER_SPECIFIED:
        return chrome.i18n.getMessage('actionUserSpecified');
      case ext.constants.Actions.GET_PASSPHRASE:
        return chrome.i18n.getMessage('actionUnlockKeyring');
    }

    return '';
  }
Returns the i18n title for the given PGP action.
@param {constants.Actions} action The PGP action that the user has
    requested.
@return {string} The i18n title.
@private