https://bugs.webkit.org/show_bug.cgi?id=85421 was a quick-fix for a bug where changes in sibling stacking order was not causing damage.
The solution in that patch was to consider stacking-order changes as "property changes" on layers. However, this is not an ideal clean correct design. The better design is for damage tracker to actually track changes in an order-dependent manner (assuming it turns out to be practical).
Indeed, the damage tracker is already assuming order-dependence (expanding damage due to filters assumed the layer list is already sorted in back-to-front drawing order).
moved this bug to https://code.google.com/p/chromium/issues/detail?id=230050