Bug 1051061 - [VerticalHomescreen] TypeError: this.grid._grid.dragdrop is undefined

https://bugzilla.mozilla.org/show_bug.cgi?id=1051061
I'd slightly prefer a fix that did not introduce a new variable. In order to review I had to check the entire file to make sure there was not already a variable by this name. But that's mostly a personal style issue, I think.