[Session Bookmarks] [e10s] Doesn't work correctly with enabled Electrolysis (multi-process mode)

Throws here:

``` js
            let isPrivate = "PrivateBrowsingUtils" in window
                && PrivateBrowsingUtils.isWindowPrivate(tab.linkedBrowser.contentWindow) // <---
                || tab.hasAttribute(privateAttr);
```

Also not only here because remote frame not yet initialized.
