Enable webrtc for base-browser

We disable WebRTC in Tor Browser for a few reasons:

1. UDP does not work over tor, so there's no reason to have this component
2. WebRTC in the past has had proxy-bypass bugs exposing the user's real IP
3. WebRTC fails to build for windows when using mingw rather than msvc.

With Sponsor 131, point 1 and point 2 (mostly) are no longer issues, which leaves point 3 (failing build).

There is this issue on Mozilla's bugzilla with the current latest known info about this issue:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1393901

The end goal for this issue: We want to be able to build base-browser for Windows, using mingw without the --disable-webrtc flag in the mozconfig. And of course, WebRTC features should work.

Fixing this properly will likely require changes to mingw and webrtc, and likely require some nasty debugging in Firefox on Windows.