[SPARK-20705][WEB-UI] The sort function can not be used in the master…

… page when you use Firefox or Google Chrome.

## What changes were proposed in this pull request?
When you open the master page, when you use Firefox or Google Chrom, the console of Firefox or Google Chrome is wrong. But The IE  is no problem.
e.g.
![error](https://cloud.githubusercontent.com/assets/26266482/25946143/74467a5c-367c-11e7-8f9f-d3585b1aea88.png)

My Firefox version is 48.0.2.
My Google Chrome version  is 49.0.2623.75 m.

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes #17952 from guoxiaolongzte/SPARK-20705.

core/src/main/resources/org/apache/spark/ui/static/sorttable.js

@@ -207,8 +207,8 @@ sorttable = {
 
     hasInputs = (typeof node.getElementsByTagName == 'function') &&
                  node.getElementsByTagName('input').length;
-    
-    if (node.getAttribute("sorttable_customkey") != null) {
+
+    if (node.nodeType == 1 && node.getAttribute("sorttable_customkey") != null) {
       return node.getAttribute("sorttable_customkey");
     }
     else if (typeof node.textContent != 'undefined' && !hasInputs) {