Restrict __qem usage to UA sheets

Source/core/css/parser/CSSPropertyParser.cpp

Index: Source/core/css/parser/CSSPropertyParser.cpp
diff --git a/Source/core/css/parser/CSSPropertyParser.cpp b/Source/core/css/parser/CSSPropertyParser.cpp
index 96dffc7295543f0d1fcc1c1ffffd2c87c9d34458..778f7231902a33592f5e7975dfa2b9ddbe7095c3 100644
--- a/Source/core/css/parser/CSSPropertyParser.cpp
+++ b/Source/core/css/parser/CSSPropertyParser.cpp
@@ -247,8 +247,10 @@ bool CSSPropertyParser::validUnit(CSSParserValue* value, Units unitflags, CSSPar
         return false;
     case CSSPrimitiveValue::CSS_PERCENTAGE:
         return unitflags & FPercent;
-    // TODO(timloh): Restrict usage of __qem to UA sheets
     case CSSParserValue::Q_EMS:
+        if (cssParserMode != UASheetMode)
+            return false;
+    /* fallthrough intentional */
     case CSSPrimitiveValue::CSS_EMS:
     case CSSPrimitiveValue::CSS_REMS:
     case CSSPrimitiveValue::CSS_CHS: