private void checkInit() {
        if (webDriverPool == null) {
            synchronized (this) {
                webDriverPool = new WebDriverPool(poolSize);
            }
        }
    }
set sleep time to wait until load success

@param sleepTime sleepTime
@return this
webmagic-selenium/src/main/java/us/codecraft/webmagic/downloader/selenium/SeleniumDownloader.java