Stylesheet rule.cssText of content property should be quoted when the value is string.

Source/core/css/CSSValueList.cpp

Index: Source/core/css/CSSValueList.cpp
diff --git a/Source/core/css/CSSValueList.cpp b/Source/core/css/CSSValueList.cpp
index 1a6adf92aae4518e15ad96533f07ed4bc96b9ab2..c293b209ff07d8ba79856a1bb216df6621ae1430 100644
--- a/Source/core/css/CSSValueList.cpp
+++ b/Source/core/css/CSSValueList.cpp
@@ -97,7 +97,7 @@ PassRefPtr<CSSValueList> CSSValueList::copy()
     return newList.release();
 }
 
-String CSSValueList::customCssText() const
+String CSSValueList::customCssText(CssTextFormattingFlags formattingFlag) const
 {
     StringBuilder result;
     String separator;
@@ -119,7 +119,10 @@ String CSSValueList::customCssText() const
     for (unsigned i = 0; i < size; i++) {
         if (!result.isEmpty())
             result.append(separator);
-        result.append(m_values[i]->cssText());
+        if (formattingFlag == AlwaysQuoteCSSString && m_values[i]->isPrimitiveValue())
+            result.append(toCSSPrimitiveValue(m_values[i].get())->customCssText(AlwaysQuoteCSSString));
+        else
+            result.append(m_values[i]->cssText());
     }
 
     return result.toString();