scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp: moves the delegate from the old scrollerImp to the new one, and also happens to call shouldUseLayerPerPartForScrollerImp. Since scrollerImpWithStyle: has not returned yet, the scrollbarPainterForScrollbar still returns the old scrollerImp, so this assertion fires. It's safe to remove this because supportsUpdateOnSecondaryThread doesn't make use of the imp. <rdar://problem/16624332>
Created attachment 229937 [details] patch
http://trac.webkit.org/changeset/167693