See bug for discussion. Basically, the root cause to the problem is
that on the Mac, all the drag-and-drop action happens during the call
to pSlideSorterViewShell->StartDrag() in the Initialize method. Thus
no wonder that mpDragAndDropContext is null in the ProcessDragEvent()
as it has not been initialized yet, and is in fact initialised
pointlessly only after the drag-and-drop already has finished.

Reverted just for Mac OS X and ifdefified in a straightforward even if
ugly fashion.

Change-Id: Icfb62fb24a5c72fda39c8bcea125267c99ecf624