In 9x frames no longer had a the backdrop system automatically applied to all of them, you had to manually add it to frames that needed a backdrop so Blizz added the system to the GameTooltip.
In 9.1.5 the layout of the GameTooltip frame changed so they can have funkier tooltips. With this change, the backdrop was moved from being applied to the top level frame (GameTooltip) to the NineSlice sub-frame so:
Code:
GameTooltip:GetBackdrop()
changed to:
Code:
GameTooltip.NineSlice:GetBackdrop()