把 StyleSheet 的内容转换成 css 字符串
@param  {StyleSheet} styleSheet
@return {String} css 字符串
function styleSheetToString(styleSheet) {
    var result = "";
    var rules = styleSheet.cssRules,
        rule;
    for (var i = 0; i < rules.length; i++) {
        rule = rules[i];
        if (rule instanceof CSSOM.CSSImportRule) {
            result += styleSheetToString(rule.styleSheet) + '\n';
        } else {
            result += rule.cssText + '\n';
        }
    }
    return result;
}