Preserve identical values for background-size and -webkit-mask-size.

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 5aeb32dc8501d73e57db9d6ea40fa14c7cbe8989..c9c995bc63ba21fde79a60f8bb4d30c005a7ed7d 100644
--- a/Source/core/css/parser/CSSPropertyParser.cpp
+++ b/Source/core/css/parser/CSSPropertyParser.cpp
@@ -2852,7 +2852,11 @@ PassRefPtrWillBeRawPtr<CSSValue> CSSPropertyParser::parseFillSize(CSSPropertyID
 
     if (!parsedValue2)
         return parsedValue1;
-    return createPrimitiveValuePair(parsedValue1.release(), parsedValue2.release());
+
+    Pair::IdenticalValuesPolicy policy = propId == CSSPropertyWebkitBackgroundSize ?
+        Pair::DropIdenticalValues : Pair::KeepIdenticalValues;
+
+    return createPrimitiveValuePair(parsedValue1.release(), parsedValue2.release(), policy);
 }
 
 bool CSSPropertyParser::parseFillProperty(CSSPropertyID propId, CSSPropertyID& propId1, CSSPropertyID& propId2,