Sites for testing:

https://www.barnesandnoble.com/#

https://trello.com/signup?returnUrl=/

https://osu.ppy.sh/p/register

https://global.account.xiaomi.com/pass/register

For testing, you have to disable the following feature:
 --disable-features=AutofillRestrictUnownedFieldsToFormlessCheckout

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19ef22b3413bd4fbe36a88506ab8212fe31ba6c1

commit 19ef22b3413bd4fbe36a88506ab8212fe31ba6c1
Author: Maxim Kolosovskiy <kolos@chromium.org>
Date: Thu Apr 19 16:31:00 2018

[Password Manager] Allow password generation for forms w/o <form> tag

Bug:  834749 
Change-Id: I94d0614bf8adf8b4ba841da1cea02cb02efd0fb9
Reviewed-on: https://chromium-review.googlesource.com/1019400
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552042}
[modify] https://crrev.com/19ef22b3413bd4fbe36a88506ab8212fe31ba6c1/chrome/renderer/autofill/password_generation_agent_browsertest.cc
[modify] https://crrev.com/19ef22b3413bd4fbe36a88506ab8212fe31ba6c1/components/autofill/content/renderer/password_generation_agent.cc