scrollHeight

scrollHeight=显示内容高度+隐藏内容高度

参考：

https://developer.mozilla.org/en-US/docs/Web/API/Element.scrollHeight

http://stackoverflow.com/questions/2940006/what-is-scrolltop-and-scrollheight

http://blog.csdn.net/wayne23/article/details/1487718

http://www.cnblogs.com/panjun-Donet/articles/1294033.html