You are probably looking for a change made after 540014 (known good), but no later than 540015 (first known bad).
CHANGELOG URL:
The script might not always return single CL as suspect as some perf builds might get missing due to failure.
https://chromium.googlesource.com/chromium/src/+log/21d0f03dc419aa224e5d7425469f846ed6287184..88b907593672e1530c42200484e4c604d75dbcdf

Suspecting: https://chromium.googlesource.com/chromium/src/+/88b907593672e1530c42200484e4c604d75dbcdf

@aee:‪ Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.‬

This is hard to replicated. I see the settings controlled by the extensions on my corp installation of chrome, but I can't replicate with an extension from the webstore. I'll continue trying to find an example that I can apply to a fresh install.

Would an okay solution be to use the previous tooltip text if chrome.settingsPrivate.PrefObject.controlledByName is not available or empty for the preference?

The previous version text was "This setting is enforced by an extension."

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7

commit 8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7
Author: Esmael El-Moslimany <aee@chromium.org>
Date: Tue Mar 06 22:18:24 2018

Settings WebUI: show fallback tooltip text when extension name is not available from the preference

Bug:  819123 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I35712fb750e941ff14801c41441289b2591911dd
Reviewed-on: https://chromium-review.googlesource.com/951898
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541217}
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/chrome/app/generated_resources.grd
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/chrome/browser/resources/settings/settings_ui/settings_ui.js
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/chrome/browser/ui/webui/policy_indicator_localized_strings_provider.cc
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/chrome/test/data/webui/cr_elements/cr_policy_indicator_behavior_tests.js
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/chrome/test/data/webui/cr_elements/cr_policy_strings.js
[modify] https://crrev.com/8a7e4e1d1e06ef41632a05c588c593d8cc6ce3d7/ui/webui/resources/cr_elements/policy/cr_policy_indicator_behavior.js

 Issue 845574  has been merged into this issue.