stevenjb@ - do you know who wrote the Auto Open Downloads setting? This would probably be best assigned to him/her. 

Well, I'm not sure what you mean by "wrote the Auto Open Downloads settings", but git-blame shows that you most recently worked on the JS/HTML and dpapad@ and tommycli@ wrote the bulk of the handler code:
https://chromium.googlesource.com/chromium/src/+blame/c9d00972713257c6cf9394686433f4a9002ba418/chrome/browser/ui/webui/settings/downloads_handle

The regression was in the setting not the feature, so the test responsibility would be with someone involved in writing the Settings UI.

Sorry- What I meant by "wrote the Auto Open Downloads settings" was simply that the tests that are missing would probably be easiest to write for whoever added the setting for Auto Open Download originally. My recent changes to the Downloads settings have been to add the Network File Share subpage and corresponding tests.

Sorry, my link above appears to be incomplete, but what I was trying to suggest was:

Look at the source for the code you think needs the test, e.g.
https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/settings/downloads_handler.cc

Select 'View in' > 'Git blame' to find authors.

I am really not at all familiar with this page.