Fix serialization of css-wide keywords in custom properties

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/CSSCustomPropertyDeclaration.h"
 
 #include "core/css/parser/CSSParserTokenRange.h"
 
 namespace blink {
 
 DEFINE_TRACE_AFTER_DISPATCH(CSSCustomPropertyDeclaration)
 {
     CSSValue::traceAfterDispatch(visitor);
 }
 
 String CSSCustomPropertyDeclaration::customCSSText() const
 {
     if (m_value)
         return m_value->tokenRange().serialize();
-    return emptyString();
+    DCHECK(m_valueId != CSSValueInternalVariableValue);
+    return getValueName(m_valueId);
 }
 
 } // namespace blink