Fix for bug 46223, not being able to accept drops from other applications. Basically,

when a drag came in from outside, the drag service was never being notified and a drag
session was never being created. Fix is to use the Start/EndDragSession() methods which
were designed for just this situation. r=hyatt

widget/src/windows/nsNativeDragTarget.cpp

@@ -217,6 +217,11 @@ STDMETHODIMP nsNativeDragTarget::DragEnter(LPDATAOBJECT pIDataSource,
   if (DRAG_DEBUG) printf("DragEnter\n");
 
 	if (mDragService) {
+
+    // tell the drag service about this drag (it may have come from an
+    // outside app).
+    mDragService->StartDragSession();
+
     // We have a new IDataObject, release the old one, if necessary and
     // keep a pointer to the new on
     NS_IF_RELEASE(mDataObj);
@@ -261,6 +266,10 @@ STDMETHODIMP nsNativeDragTarget::DragLeave() {
 
   if (DRAG_DEBUG) printf("DragLeave\n");
 	if (mDragService) {
+
+    // tell the drag service that we're done with it
+    mDragService->EndDragSession();
+
     // tell anyone interested to stop tracking drags, but only when we're
     // leaving a window, not a child widget
     nsCOMPtr<nsIWidget> parent ( dont_AddRef(mWindow->GetParent()) );