Web developer network tab breaks first-party isolation in some cases II

There are rare cases where the first-part isolation breaks if the Web developer Network tab is open. This got first reported on our blog: ​https://blog.torproject.org/blog/tor-browser-65a5-released#comment-224102

Steps to reproduce (works in the alpha series on Windows at least):

1. Start a fresh Tor Browser and set the Torbutton log level to "3"
2. Open the Network tab in the Web developer console (Ctrl + Shift + Q)
3. Go to ​https://torproject.org
4. Reload the page with the arrow in the URL bar

Result:

Torbutton INFO: tor SOCKS: https://www.torproject.org/static/css/bootstrap.css.map via--unknown--:878a267349f5b487247d0a0175ae27f2

It is actually only the request for one resource that is affected. And having the Network tab open is crucial for reproducing the bug.