constructor(actionExecutor, content, preferences, errorCallback) {
    super(
        chrome.i18n.getMessage('promptEncryptSignTitle'), content,
        errorCallback);

    this.actionExecutor_ = actionExecutor;
    this.errorCallback_ = errorCallback;

    /**
     * A holder for the intended recipients of a PGP message.
     * @type {?panels.ChipHolder}
     * @private
     */
    this.chipHolder_ = null;

    /**
     * User preferences.
     * @type {e2e.ext.Preferences}
     * @private
     */
    this.preferences_ = preferences;
  }
@param {!e2e.ext.actions.Executor} actionExecutor Executor for the
    End-to-End actions.
@param {!e2e.ext.messages.BridgeMessageRequest} content The content that
    the user wants to encrypt.
@param {!e2e.ext.Preferences} preferences User preferences.
@param {function(Error)} errorCallback A callback where errors will be
    passed to.