renderKeyringPassphrase_(elem, contentBlob) {
    var dialog = new dialogs.Generic(
        '',
        goog.bind(
            function(passphrase) {
              try {
                // Correct passphrase entered.
                this.pgpLauncher_.start(passphrase);
                if (contentBlob && contentBlob.action) {
                  if (contentBlob.action == constants.Actions.GET_PASSPHRASE) {
                    contentBlob.action =
                        undefined;  // Fall into the default action.
                  }
                  this.processSelectedContent_(contentBlob, contentBlob.action);
                } else {
                  this.close();
                }
              } catch (e) {  // Incorrect passphrase, so ask again.
                this.processSelectedContent_(
                    contentBlob, ext.constants.Actions.GET_PASSPHRASE);
              }
              goog.dispose(dialog);
            },
            this),
        // Use a password field to ask for the passphrase.
        dialogs.InputType.SECURE_TEXT,
        chrome.i18n.getMessage('actionEnterPassphraseDescription'),
        chrome.i18n.getMessage('actionEnterPassphrase'),
        chrome.i18n.getMessage('actionCancelPgpAction'));

    elem.textContent = '';
    this.addChild(dialog, false);
    dialog.render(elem);
    goog.dom.classlist.remove(elem, constants.CssClass.TRANSPARENT);
  }
Renders the UI elements needed for requesting the passphrase of the PGP
keyring.
@param {Element} elem The element into which the UI elements are to be
    rendered.
@param {?messages.BridgeMessageRequest} contentBlob The content that the
    user has selected.
@private