Polymer 2: Make sure that querySelectorAll works with NodeList

https://polymer-library.polymer-project.org/2.0/docs/upgrade#polymer-dom-apis

Change-Id: Id6dd6fbbdb5a9494485b6c6f5361d12306cd052c

polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js

diff --git a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
index 7d109633..43afdf8 100644
--- a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
+++ b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.js
@@ -41,8 +41,9 @@
      * @return {!Array<string>}
      */
     _getDisplayedColumns() {
-      return Polymer.dom(this.root)
-          .querySelectorAll('.checkboxContainer input:not([name=number])')
+      // Polymer2: querySelectorAll returns NodeList instead of Array.
+      return Array.from(Polymer.dom(this.root)
+          .querySelectorAll('.checkboxContainer input:not([name=number])'))
           .filter(checkbox => checkbox.checked)
           .map(checkbox => checkbox.name);
     },