Make quoteCSSString an internal helper

Source/core/html/HTMLElement.cpp

Index: Source/core/html/HTMLElement.cpp
diff --git a/Source/core/html/HTMLElement.cpp b/Source/core/html/HTMLElement.cpp
index eb8e0c7ca2c98a9cd6c1e6d0991f66de957c7ae5..ae3f9f2e9dbb82bb0dedbf4dd456c17782a225bd 100644
--- a/Source/core/html/HTMLElement.cpp
+++ b/Source/core/html/HTMLElement.cpp
@@ -145,7 +145,7 @@ void HTMLElement::mapLanguageAttributeToLocale(const AtomicString& value, Mutabl
 {
     if (!value.isEmpty()) {
         // Have to quote so the locale id is treated as a string instead of as a CSS keyword.
-        addPropertyToPresentationAttributeStyle(style, CSSPropertyWebkitLocale, quoteCSSString(value));
+        addPropertyToPresentationAttributeStyle(style, CSSPropertyWebkitLocale, serializeString(value));
 
         // FIXME: Remove the following UseCounter code when we collect enough
         // data.