Add null check for IncognitoNotificationServiceTest

#testCloseAllIncognitoNotificationIsDisplayed.

StatusBarNotification#getTag method can return null if no tag was
assigned to a notification.

This CL adds the null check here before comparing the tag with the
incognito notification tag.

Bug: 1134241
Change-Id: I548ec82aebbd15a2e837cf00316d30955d828510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463573Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Rohit Agarwal <roagarwal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816486}

chrome/android/javatests/src/org/chromium/chrome/browser/incognito/IncognitoNotificationServiceTest.java

@@ -206,8 +206,8 @@ public class IncognitoNotificationServiceTest {
                 (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
         boolean isIncognitoNotificationDisplayed = false;
         for (StatusBarNotification statusBarNotification : nm.getActiveNotifications()) {
-            if (statusBarNotification.getTag().equals(
-                        IncognitoNotificationManager.INCOGNITO_TABS_OPEN_TAG)) {
+            if (IncognitoNotificationManager.INCOGNITO_TABS_OPEN_TAG.equals(
+                        statusBarNotification.getTag())) {
                 isIncognitoNotificationDisplayed = true;
             }
         }