This is actually a common pattern in RenderLayer to do 2 traversals (one to get the root() and the other one to compute our position relative to it). By tweaking the way we traverse the tree, we should be able to squash those 2 traversals in one.
This function was removed in following commit: https://github.com/WebKit/WebKit/commit/cfe4349f145ac534cdd1701c5b97eb090630bfb6 I don't think we need to fix this bug. @Alan / @Simon - Anything further needed or we can mark this as 'RESOLVED CONFIGURATION CHANGED'?