b=497498 fire NS_DRAGDROP_DRAG at source even before synthetic NS_DRAGDROP_OVER events r=enndeakin

--HG--
extra : rebase_source : 5c3d7057b753c76039e8cb1449f9cdfbb3be4725

widget/gtk2/nsWindow.cpp

@@ -3491,6 +3491,8 @@ nsWindow::OnDragDropEvent(GtkWidget *aWidget,
     dragServiceGTK->TargetSetLastContext(aWidget, aDragContext, aTime);
     dragServiceGTK->SetCanDrop(false);
 
+    dragService->FireDragEventAtSource(NS_DRAGDROP_DRAG);
+
     nsDragEvent event(true, NS_DRAGDROP_OVER, innerMostWidget);
 
     InitDragEvent(event);