Mark reftest for bug 456147 as failing on Mac; problem isn't caused by this bug but by an unknown earlier regression

layout/reftests/bugs/reftest.list

@@ -919,4 +919,5 @@ fails == 441259-2.html 441259-2-ref.html # bug 441400
 == 451168-1.html 451168-1-ref.html
 == 454361.html about:blank
 == 455280-1.xhtml 455280-1-ref.xhtml
-== 456147.xul 456147-ref.html
+fails-if(MOZ_WIDGET_TOOLKIT=="cocoa") == 456147.xul 456147-ref.html # bug 456147, but not caused by it
+