Implement unset value handling

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 451 matching lines @@
 
     CSSValueID id = value->id;
 
     int num = inShorthand() ? 1 : m_valueList->size();
 
     if (id == CSSValueInherit) {
         if (num != 1)
             return false;
         addExpandedPropertyForValue(propId, cssValuePool().createInheritedValue(), important);
         return true;
-    }
-    else if (id == CSSValueInitial) {
+    } else if (id == CSSValueInitial) {
         if (num != 1)
             return false;
         addExpandedPropertyForValue(propId, cssValuePool().createExplicitInitialValue(), important);
         return true;
+    } else if (id == CSSValueUnset) {
+        if (num != 1)
+            return false;
+        addExpandedPropertyForValue(propId, cssValuePool().createUnsetValue(), important);
+        return true;
     }
 
     if (CSSParserFastPaths::isKeywordPropertyID(propId)) {
         if (!CSSParserFastPaths::isValidKeywordPropertyAndValue(propId, id))
             return false;
         if (m_valueList->next() && !inShorthand())
             return false;
         addProperty(propId, cssValuePool().createIdentifierValue(id), important);
         return true;
     }
@@ skipping 7983 matching lines @@
             return nullptr;
         a = args->next();
 
         argNumber++;
     }
 
     return transformValue.release();
 }
 
 } // namespace blink