[Downloads] Re-add notification namespace

The namespace accidentally got removed while experimenting with a fix for
notification grouping.

BUG=678694

Review-Url: https://codereview.chromium.org/2619643002
Cr-Commit-Position: refs/heads/master@{#442070}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadNotificationService.java

@@ -753,7 +753,7 @@ public class DownloadNotificationService extends Service {
      */
     @VisibleForTesting
     void updateNotification(int id, Notification notification) {
-        mNotificationManager.notify(id, notification);
+        mNotificationManager.notify(NOTIFICATION_NAMESPACE, id, notification);
     }
 
     /**