CL is currently being reviewed: https://crrev.com/c/1330899 that will allow us to group users together for the experiment.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69

commit 523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69
Author: Hector Carmona <hcarmona@chromium.org>
Date: Mon Nov 19 19:51:40 2018

Navi: Create onboarding groups that are allowed to ping finch.

Only users in a group will be able to ping finch to make retention
analysis easier.

Groups are:
  - Specified by finch
  - Incremented with each new version
  - Not reused
  - Cleared out when experiment ends

Bug:  904051 
Change-Id: I726b43cd122969447cd6d2e3e207b12a5e9c97b4
Reviewed-on: https://chromium-review.googlesource.com/c/1330899
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609403}
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/browser/prefs/browser_prefs.cc
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/browser/profiles/profile_manager.cc
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/browser/ui/startup/startup_browser_creator.cc
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/browser/ui/webui/welcome/nux_helper.cc
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/browser/ui/webui/welcome/nux_helper.h
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/common/pref_names.cc
[modify] https://crrev.com/523cf50a9d77e4745b52e06d6c8c7ffa44fe3f69/chrome/common/pref_names.h