Bug 1799343 - Fix a crashtest that try didn't catch.

I'll remove display: -moz-popup in bug 1799580.

MANUAL PUSH: Trivial orange fix CLOSED TREE

layout/base/crashtests/405049-1.xhtml

@@ -1,3 +1,3 @@
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" style="display: table;">
-<box style="display: -moz-popup;"/>
-</window>
+  <panel/>
+</window>