Handle drop tab from other window
Workaround: can't sometimes make loading tab private
We use documented
tab.linkedBrowser
	.contentWindow
	.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
	.getInterface(Components.interfaces.nsIWebNavigation)
	.QueryInterface(Components.interfaces.nsILoadContext);
But the same bugs happens for
tab.linkedBrowser
	.webNavigation
	.QueryInterface(Components.interfaces.nsILoadContext);
- we get _another_ webNavigation object.