We are not testing performance of the stretching alignment logic, so we are not aware of possible regressions in such an important case, give that is the default value for grid items's alignment properties (align-self, justify-self).
Created attachment 255018 [details] Patch
Comment on attachment 255018 [details] Patch Clearing flags on attachment: 255018 Committed r185654: <http://trac.webkit.org/changeset/185654>
All reviewed patches have been landed. Closing bug.