Reviewed by Alexey Proskuryakov.

        Rename DragController::dragOperation() to sourceDragOperation() for clarity
        Remove unused setDragOperation()

        There are two times DragOperation is used.  The source specifies
        what drag operations it supports (in a mask) and the destination
        picks a subset of those as DragOperations it supports if a drop were
        to occur.  I was confused when I first saw this accessor as to which
        it meant.  It turns out that this is only used when WebKit starts a
        drag, and we keep it around so that we can provide this information
        to other Mac OS X applications when they ask.  I've renamed the method
        for clarity so the next person in this code will be less confused.

        No functional changes, thus no tests.

        * page/DragController.h:
        (WebCore::DragController::sourceDragOperation):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43967 268f45cc-cd09-0410-ab3c-d52691b4dbfc