Disable reftest 508908-1.xul. See bug 556124.

layout/reftests/bugs/reftest.list

@@ -1353,7 +1353,8 @@ fails-if(MOZ_WIDGET_TOOLKIT!="cocoa") == 488692-1.html 488692-1-ref.html # needs
 == 507762-2.html 507762-2-ref.html
 == 507762-3.html 507762-1-ref.html
 == 507762-4.html 507762-2-ref.html
-== 508908-1.xul 508908-1-ref.xul
+# see bug 556124
+fails == 508908-1.xul 508908-1-ref.xul
 == 508919-1.xhtml 508919-1-ref.xhtml
 == 509155-1.xhtml 509155-1-ref.xhtml
 == 512410.html 512410-ref.html