Spoke to Estelle and Lindsay about this. Lindsay mentioned that tests don't have to be complete by feature freeze, but both agreed that it would be ideal if they were.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a1dc0c202dcb3ba7d5101a7c09c55892e994c7f5

commit a1dc0c202dcb3ba7d5101a7c09c55892e994c7f5
Author: Gregory Chatzinoff <gchatz@chromium.org>
Date: Wed Jan 03 01:55:21 2018

Write EG Tests for New Tab Tip promotion.

Bug: 767219
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I677343a47696ed1b4a70dd9c029f2b65fe5d9181
Reviewed-on: https://chromium-review.googlesource.com/843462
Commit-Queue: Gregory Chatzinoff <gchatz@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526579}
[modify] https://crrev.com/a1dc0c202dcb3ba7d5101a7c09c55892e994c7f5/ios/chrome/browser/feature_engagement/BUILD.gn
[modify] https://crrev.com/a1dc0c202dcb3ba7d5101a7c09c55892e994c7f5/ios/chrome/browser/feature_engagement/feature_engagement_egtest.mm

This bug is now tracking just EG tests for incognito promo. These tests probably can't be written until the feature engagement tracker issues are worked out, since it would begin tracking incognito events for test builds.