Sets a cookie given a valid URL and explicit user-provided cookie attributes. This function expects each attribute to be well-formed. It will check for disallowed characters (e.g. the ';' character is disallowed within the cookie value attribute) and fail without setting the cookie if such characters are found. If |callback| is non-NULL it will be executed asnychronously on the UI thread after the cookie has been set. Returns false (0) if an invalid URL is specified or if cookies cannot be accessed.