Code:
GameTooltip.NineSlice:GetBackdrop()
Although it's not really going to solve any problems caused by the underlying change.
There is a GameToolTip widget type (like a Frame or a Button widget type) and there is the "GameToolTip" frame (a GameToolTip widget that has been given the name "GameToolTip") that inherits "SharedTooltipTemplate" which has a sub-frame that inherits the "NineSlicePanelTemplate". This is new to 9.1.5.