Audit path from browser making URL requests down to contacting SOCKS proxy

[riastradh-brave]

We need to make sure that all paths from making a URL request in browser-laptop down to the actual network activity leads only to network activity with the configured proxy, at least for Tor sessions. For example, when a SOCKS5 proxy is configured, there should be no DNS requests (see, e.g., brave/muon#164). This is not trivial because there are many such paths!

Further, we should have a practice in place to make sure the audit happens again any time it would be invalidated, e.g. when we update Chromium.

[transcribed from https://github.com/brave/muon/issues/465]