Add `privacy.network.webRTCNonProxiedUdpTransportEnabled` to extensions API

Expose the preference to extensions' privacy API. Disabling this preference prevents non-proxied UDP to be used as it leads to IP leak when using WebRTC. Currently, this is effectively disabling all UDP traffic until UDP supporting proxy is available in chrome.

This is based on an ongoing CL https://codereview.chromium.org/1309543004

BUG=webrtc:4784

Review URL: https://codereview.chromium.org/1315223002

Cr-Commit-Position: refs/heads/master@{#346786}
4 files changed