Change Speed of Clip with Scripting

I'm trying to create a shortcut key that turns on/off backward playback of the selected TrackItem.

(Because there is no corresponding option in 'Command' in keyboard shortcut settings)

function ReverseTrackItem(trackItem) {
    var orgStart = trackItem.start;    // if 10
    var orgEnd = trackItem.end;        // if 20

    // Reverse ?
    trackItem.start = orgEnd;          // result 20
    trackItem.end = orgStart;          // result 20
}

I tried this, but it doesn't work as expected. 

If there isn't an API yet, is there another way that doesn't go through the API?