Add missing SetLevel call that is caught by assertions once the following patch lands. (Bug 492675) r=bzbarsky

layout/style/nsStyleSet.cpp

@@ -750,6 +750,7 @@ nsStyleSet::WalkRestrictionRule(nsIAtom* aPseudoType)
 {
   // This needs to match GetPseudoRestriction in nsRuleNode.cpp.
   if (aPseudoType) {
+    mRuleWalker->SetLevel(eAgentSheet, PR_FALSE);
     if (aPseudoType == nsCSSPseudoElements::firstLetter)
       mRuleWalker->Forward(mFirstLetterRule);
     else if (aPseudoType == nsCSSPseudoElements::firstLine)