added support for DSCP marking outbound media packets on Chromium

[jbg]

Chromium-based browsers support DSCP marking of outbound media packets, but it's disabled by default because some middleboxes on the wider Internet might drop packets with DSCP marks set. On controlled networks (e.g. corporate networks) DSCP can be very helpful for prioritising videoconferencing traffic.

This patch allows it to optionally be enabled with a enableDSCP option.

Relevant community discussion: https://community.jitsi.org/t/looking-for-a-way-to-configure-udp-ports-used-in-peer-to-peer-for-1-to-1-sessions-p2p4121/103706/6