Fix serialization of content property to always quote

Source/core/css/CSSValuePool.cpp

Index: Source/core/css/CSSValuePool.cpp
diff --git a/Source/core/css/CSSValuePool.cpp b/Source/core/css/CSSValuePool.cpp
index bd6950eeb9b2978f879650b0fc182dd303707265..a7976602416703379594ab3ef0be9c6aeea3edb4 100644
--- a/Source/core/css/CSSValuePool.cpp
+++ b/Source/core/css/CSSValuePool.cpp
@@ -130,7 +130,7 @@ PassRefPtrWillBeRawPtr<CSSPrimitiveValue> CSSValuePool::createFontFamilyValue(co
 {
     RefPtrWillBeMember<CSSPrimitiveValue>& value = m_fontFamilyValueCache.add(familyName, nullptr).storedValue->value;
     if (!value)
-        value = CSSPrimitiveValue::create(familyName, CSSPrimitiveValue::CSS_STRING);
+        value = CSSPrimitiveValue::create(familyName, CSSPrimitiveValue::CSS_CUSTOM_IDENT);
     return value;
 }