Bug 1599996 - Re-introduce a minimum width to commonDialog.xul. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D55170

--HG--
extra : moz-landing-system : lando

toolkit/components/prompts/content/commonDialog.css

@@ -2,6 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+:root {
+  min-width: 29em;
+}
+
 #dialogGrid {
   display: grid;
   grid-template-columns: auto 1fr;