Apparently, it doesn't do anything.
Created attachment 331652 [details]
Putting for review in case anyone knows of more details here. I couldn't quite follow the code to confirm what exactly "not used means", and which OS versions that is the case with.
Comment on attachment 331652 [details]
Clearing flags on attachment: 331652
Committed r227172: <https://trac.webkit.org/changeset/227172>
All reviewed patches have been landed. Closing bug.