Only Chromium wanted this, for no particular reason.
There are no tests that change this setting (except for resetting it to true, which it is already), and it can't work on some ports, because it's too late to change scrollbar type after a view is created.
Created attachment 230015 [details]
What about WebKitTestRunner!?
Oh, I guess its call to enable scrollbars is shared, nevermind.
Comment on attachment 230015 [details]
Clearing flags on attachment: 230015
Committed r167732: <http://trac.webkit.org/changeset/167732>
All reviewed patches have been landed. Closing bug.