Mark failing reftest as random-if(cocoaWidget), a=bustage

layout/reftests/bugs/reftest.list

@@ -1395,7 +1395,7 @@ needs-focus fails == 503531-1.html 503531-1-ref.html
 == 507762-2.html 507762-2-ref.html
 == 507762-3.html 507762-1-ref.html
 == 507762-4.html 507762-2-ref.html
-== 508816-1.xul 508816-1-ref.xul
+random-if(cocoaWidget) == 508816-1.xul 508816-1-ref.xul # Bug 631982
 == 508816-2.html 508816-2-ref.html
 == 508908-1.xul 508908-1-ref.xul
 == 508919-1.xhtml 508919-1-ref.xhtml