Bug 1227501 patch 1 - Don't check flags for eCSSPropertyExtra_variable. r=xidorn

I tested locally that the crashtest in patch 2 hits the fatal assertion
without this patch, and passes with the patch.

--HG--
extra : commitid : 8NwOfppKxBN

layout/style/nsRuleNode.cpp

@@ -5945,6 +5945,7 @@ nsRuleNode::ComputeDisplayData(void* aStartStruct,
           nsCSSProps::LookupProperty(buffer,
                                      nsCSSProps::eEnabledForAllContent);
         if (prop != eCSSProperty_UNKNOWN &&
+            prop != eCSSPropertyExtra_variable &&
             nsCSSProps::PropHasFlags(prop,
                                      CSS_PROPERTY_CREATES_STACKING_CONTEXT))
         {