Desktop's `nsIBrowserWindow.createContetnWindow` will return null when
creating new window. Then browsing context is observed by `nsFrameLoader`.
Actually, GeckoView's `createContentWindow` always creates new window.
So if using new window option, we should return null like desktop.
Also, if no callback, we return browsing context even if creating new window
due to no way to notify browsing context. And,
`nsIWindowProvider.provideWindow` doesn't use the callback, we have to return
browsing context for this situation.
Differential Revision: https://phabricator.services.mozilla.com/D197131
ea3bbc79e9