Step 1: duplicate layer.
Step 2: On the upper layer, select the whitespace, then select -> modify -> expand. 1 usually works, but for this one I went with 2 or 3, I forget which.
Step 3: Set black as the background color, then ctrl+delete to delete-and-leave-behind-background-color.
Step 4: Set the upper layer invisible, then select the whitespace in the lower layer. Don't expand it again.
Step 5: Set the upper layer visible and the lower layer invisible, make sure the upper layer is selected, and just hit del.
Step 6: PNG!