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

Source/core/css/parser/CSSPropertyParser.cpp

 /*
  * Copyright (C) 2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2012 Adobe Systems Incorporated. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
@@ skipping 2834 matching lines @@
         }
     } else if (!parsedValue2 && propId == CSSPropertyWebkitBackgroundSize) {
         // For backwards compatibility we set the second value to the first if it is omitted.
         // We only need to do this for -webkit-background-size. It should be safe to let masks match
         // the real property.
         parsedValue2 = parsedValue1;
     }
 
     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,
     RefPtrWillBeRawPtr<CSSValue>& retValue1, RefPtrWillBeRawPtr<CSSValue>& retValue2)
 {
     RefPtrWillBeRawPtr<CSSValueList> values = nullptr;
     RefPtrWillBeRawPtr<CSSValueList> values2 = nullptr;
     CSSParserValue* val;
     RefPtrWillBeRawPtr<CSSValue> value = nullptr;
     RefPtrWillBeRawPtr<CSSValue> value2 = nullptr;
@@ skipping 5617 matching lines @@
         parsedValues->append(CSSPrimitiveValue::createIdentifier(CSSValueFill));
     if (!seenStroke)
         parsedValues->append(CSSPrimitiveValue::createIdentifier(CSSValueStroke));
     if (!seenMarkers)
         parsedValues->append(CSSPrimitiveValue::createIdentifier(CSSValueMarkers));
 
     return parsedValues.release();
 }
 
 } // namespace WebCore