Replace undefined with null in safe_browsing.js.

Bug: 1042984
Change-Id: I12c1287f42f39dfab883c2976b7c7ae39441cf7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2005906Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732925}

components/safe_browsing/content/web_ui/resources/safe_browsing.js

@@ -248,7 +248,7 @@ cr.define('safe_browsing', function() {
     const token = result[0];
     const request = result[1];
 
-    if ($(tableId + '-' + token) === undefined) {
+    if ($(tableId + '-' + token) === null) {
       insertTokenToTable(tableId, token);
     }