Shorthand serialization should check for var in longhands.

third_party/WebKit/Source/core/css/StylePropertySerializer.cpp

Index: third_party/WebKit/Source/core/css/StylePropertySerializer.cpp
diff --git a/third_party/WebKit/Source/core/css/StylePropertySerializer.cpp b/third_party/WebKit/Source/core/css/StylePropertySerializer.cpp
index ae217b6d488130767845519d84ab027f648ea01a..9f8264f52773f604f8d37cd5a99c2d9ebb68be27 100644
--- a/third_party/WebKit/Source/core/css/StylePropertySerializer.cpp
+++ b/third_party/WebKit/Source/core/css/StylePropertySerializer.cpp
@@ -400,6 +400,8 @@ String StylePropertySerializer::commonShorthandChecks(const StylePropertyShortha
         // TODO(timloh): This should also check unset
         if (value.isInheritedValue())
             return emptyString();
+        if (value.isVariableReferenceValue())
+            return emptyString();
     }
 
     return String();