The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/42385e34b4f3f7c20efff7f29afdbedf21a27654

commit 42385e34b4f3f7c20efff7f29afdbedf21a27654
Author: Charlie Harrison <csharrison@chromium.org>
Date: Mon May 14 14:09:27 2018

Remove tab-under enterprise policy

Now that tab-unders can be controlled via the popup setting, admins can
simply use the popup enterprise policy to also control tab-unders.

Since tab-under blocking was never actually launched in versions <M68,
this CL also drops policy support for them completely.

Bug:  842306 
Change-Id: Ie7b7d41b2caeaf1e26bc931777a25ec113c03dc4
Reviewed-on: https://chromium-review.googlesource.com/1056170
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Drew Wilson <atwilson@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558294}
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/policy/configuration_policy_handler_list_factory.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/prefs/browser_prefs.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/ui/blocked_content/popup_opener_tab_helper_unittest.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/ui/blocked_content/tab_under_blocker_browsertest.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/ui/blocked_content/tab_under_navigation_throttle.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/browser/ui/blocked_content/tab_under_navigation_throttle.h
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/common/pref_names.cc
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/common/pref_names.h
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/chrome/test/data/policy/policy_test_cases.json
[modify] https://crrev.com/42385e34b4f3f7c20efff7f29afdbedf21a27654/components/policy/resources/policy_templates.json