I use the crop tool with perspective box checked on my desktop, would like to use it on my iPad.
Another option would be the Warp option under the Amperstand "&" menu. You would want to move the outer frame handles out from the vertical center. It it's not possible to link them, so you would need to move each side individually and crop as an additional step afterwards.
(click images to view larger)
probably would need to move the top right frame handle out event a little more here
North America
Europe, Middle East and Africa
Asia Pacific