Adding test for bug 410621. r+sr=bzbarsky

layout/reftests/bugs/reftest.list

@@ -822,6 +822,7 @@ random == 403134-1.html 403134-1-ref.html # bug 405377
 != 409659-1b.html 409659-1-ref.html
 != 409659-1c.html 409659-1-ref.html
 == 409659-1d.html 409659-1-ref.html
+== 410621-1.html 410621-1-ref.html
 == 411059-1.html 411059-1-ref.html
 == 411334-1.xml 411334-1-ref.xml
 == 411585-1.html 411585-1-ref.html