[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 <[email protected]>
Author: 郭小龙 10207633 <[email protected]>
Author: guoxiaolongzte <[email protected]>

Closes #17952 from guoxiaolongzte/SPARK-20705.

(cherry picked from commit 99d5799)
Signed-off-by: Sean Owen <[email protected]>

Author: guoxiaolong

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) {