CL 1346400 removed some potentially-needed logic around the Document provider and unity.

I think the thing to do is very simple - just set or unset the pref - but I'm not sure how we choose pre- or post- unity logic, or if that's even required. 

Creating this bug to readd since this will likely slip right behind branch point, either due to a correction or (more likely) some extra caution and review getting it landed the first time.

See also 908448