Use an OS-chosen port number for tor socks proxy

[riastradh-brave]

Currently, in browser-laptop, we use port 9250, 9260, 9270, 9280, or 9290 for the tor socks proxy, depending on the release channel. This allows a standard release of Brave to coexist with a beta release to coexist with a local development build, but not more than one of each kind at a time, and if anything happens to be using the port number when Brave starts, it just fails. We should let the tor daemon ask the OS to choose an unused port number and use that instead.

This requires an extra step from what we currently do in browser-laptop. Specifically, currently, we:

1. Pick the port number, say 9250.
2. Create a Tor browser context with a Tor socks proxy config pointing at socks5://127.0.0.1:9250.
   • (The Tor socks proxy config is customized to use a unique username and password per first-party origin, per per fresh load.)
3. Start tor with --socksport 9250.

Instead, we should:

1. Create a Tor browser context with an uninitialized Tor socks proxy config that blocks all requests.
2. Start tor with --socksport 0.
3. Learn the port number from the tor control channel: GETINFO net/listeners/socks.
4. Configure the Tor socks proxy config with the port number we learn from that.

[btlechowski]

Verification passed on

Brave	1.16.50 Chromium: 86.0.4240.55 (Official Build) dev (x86_64)
Revision	a6d625ef6f7fe8ea0675f1cf759155a05ee1be40-refs/branch-heads/4240@{#953}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#4802

Verified OS-chosen port for tor
Opened Tor Window the first time

Opened Tor Window the second time

Rest of the verification done in #359 (comment)

---

Verified passed with

Brave	1.16.55 Chromium: 86.0.4240.72 (Official Build) dev (x86_64)
Revision	581582174c512f44f44fd1aea340471f54b2365f-refs/branch-heads/4240@{#1134}
OS	macOS Version 10.14.6 (Build 18G3020)

Verified test plan from brave/brave-core#4802 - notes can be found under #359 (comment)

---

Verification passed on

Brave | 1.16.55 Chromium: 86.0.4240.72 (Official Build) dev (64-bit)
-- | --
Revision | 581582174c512f44f44fd1aea340471f54b2365f-refs/branch-heads/4240@{#1134}
OS | Windows 10 OS Version 1903 (Build 18362.1082)

Verified test plan from brave/brave-core#4802 - notes can be found under #359 (comment)