The GetSessionStartupPref method in StartupBrowserCreator is deceptive: it's actually hiding some policy logic and may modify the contents of the pref.

Some of that logic is crufty and obsolete and can be removed. Other bits are still relevant but probably belong in a broader behavior-checking function. And none of it is unit-tested.

Unfortunately, there are ~4 callers of this method outside SBC/SBCImpl, so there will be a little extra legwork.