RenderGrid::computeNormalizedFractionBreadth() is supposed to get as fourth argument either the grid element's content box size or the max-size of a particular grid item. For the former we were incorrectly passing the free space after distributing the available space to tracks.
Apart from that, that method has a bug in its implementation. As we were using the free space instead of the grid element's content box size, we didn't have to subtract the usedBreadth of grid tracks from the passed in value. That does not work for the case of the caller passing the max-size of a particular item.
Created attachment 244902 [details]
Comment on attachment 244902 [details]
Committed r178701: <http://trac.webkit.org/changeset/178701>