Bug 1823489 - Have the migration-wizard element create the panel-list in its shadow root. r=kpatenio,negin,desktop-theme-reviewers,dao

This involves opening the shadow root of the migration wizard, as the panel-list
really isn't designed to handle being embedded within a closed shadow root.

Differential Revision: https://phabricator.services.mozilla.com/D182484

browser/components/preferences/main.js

@@ -2157,12 +2157,6 @@ var gMainPane = {
     if (!migrationWizardDialog.firstElementChild) {
       let wizard = document.createElement("migration-wizard");
       wizard.toggleAttribute("dialog-mode", true);
-
-      let panelList = document.createElement("panel-list");
-      let panel = document.createXULElement("panel");
-      panel.appendChild(panelList);
-      wizard.appendChild(panel);
-
       migrationWizardDialog.appendChild(wizard);
     }
     migrationWizardDialog.firstElementChild.requestState();