Bug 987680. Overlay scrollbars should be placed on top of content. r=roc

Bug 926292 accidentally put them in the background list if there were no positioned descendents that they needed to be on top of it (because this was where non-overlay scrollbars went, which never made sense). If there are no positioned descendents overlay scrollbars should go on the top-most non-positioned list, which is outlines for the current implementation.