This comes from bug 128271. We left that one for the generic issue of properties exposed for runtime disabled features. We'll use this for bringing the feature flag back that will help packagers not to ship grid layout if they aren't interested.
See also this email thread in the mailing list: https://lists.webkit.org/pipermail/webkit-dev/2014-February/026218.html
Created attachment 224870 [details]
Committed r164659: <http://trac.webkit.org/changeset/164659>