Bug 1087536 patch 2 - Assert when nsStyleSet::RuleNodeWithReplacement finds the rules in an incorrect order. r=birtles

I confirmed that this assertion fires (along with the other failures)
when running layout/style/test/test_transitions_events.html with patch 3
but not patch 1.

layout/style/nsStyleSet.cpp

@@ -1524,6 +1524,10 @@ nsStyleSet::RuleNodeWithReplacement(Element* aElement,
     }
   }
 
+  NS_ASSERTION(rulesIndex == 0,
+               "rules are in incorrect cascading order, "
+               "which means we replaced them incorrectly");
+
   return ruleWalker.CurrentNode();
 }