Bug 978997 - Fix a regression caused by bug 959847 where the root layer sometimes doesn't get an APZC. r=tnikkel on a CLOSED TREE

Root scroll frames in display root documents don't get a scroll info layer from ScrollFrameHelper::BuildDisplayList, so we always need to call RecordFrameMetrics from nsDisplayList::PaintForFrame.
Merge m-c to m-i
Automatic merge