Fix serialization of content property to always quote

LayoutTests/fast/css/content-language-with-whitespace.html

Index: LayoutTests/fast/css/content-language-with-whitespace.html
diff --git a/LayoutTests/fast/css/content-language-with-whitespace.html b/LayoutTests/fast/css/content-language-with-whitespace.html
index a88c8111a0649591696b512bdf0e51607e14a68a..ce893ee92aca261a06ae19eddfbc3ce56ab5c3bb 100644
--- a/LayoutTests/fast/css/content-language-with-whitespace.html
+++ b/LayoutTests/fast/css/content-language-with-whitespace.html
@@ -24,8 +24,8 @@ 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>
 </body>
 </html>