Fix serialization of content property to always quote

LayoutTests/fast/css/content-language-late.html

Index: LayoutTests/fast/css/content-language-late.html
diff --git a/LayoutTests/fast/css/content-language-late.html b/LayoutTests/fast/css/content-language-late.html
index 5fa3ec7d726492c74e0ff0107cfb5b55aff67b7f..b3c548d35ce91d8ddaaa1e0eb02d30b2cf1ad3d7 100644
--- a/LayoutTests/fast/css/content-language-late.html
+++ b/LayoutTests/fast/css/content-language-late.html
@@ -20,8 +20,8 @@ function languageOfNode(id) {
     var element = document.getElementById(id);
     return window.getComputedStyle(element).webkitLocale;
 }
-shouldBeEqualToString("languageOfNode('x')", "ja-JP");
-shouldBeEqualToString("languageOfNode('y')", "ar");
+shouldBeEqualToString("languageOfNode('x')", "'ja-JP'");
+shouldBeEqualToString("languageOfNode('y')", "'ar'");
 </script>
 <meta http-equiv="content-language" content="ja-JP">
 </body>