Mode 4 of WebRTC IP Address Handling Requirements (https://tools.ietf.org/html/draft-ietf-rtcweb-ip-handling-09#section-5.2):

"[...] when the application's HTTP traffic is sent through a proxy, WebRTC media traffic MUST also be proxied. If the proxy does not support UDP (as is the case for all HTTP and most SOCKS [RFC1928] proxies), or the WebRTC implementation does not support UDP proxying, the use of UDP will be disabled, and TCP will be used to send and receive media through the proxy."

Chromium extensions can implement mode 4 by setting webRTCIPHandlingPolicy to "disable_non_proxied_udp" (https://developer.chrome.com/extensions/privacy#type-IPHandlingPolicy). This currently disables all UDP connections in WebRTC (https://cs.chromium.org/chromium/src/content/renderer/p2p/port_allocator.cc?type=cs&sq=package:chromium&g=0&l=40), because we do not support UDP proxying.

This issue tracks proxied UDP support in WebRTC in Chromium. When it is complete, extensions setting disable_non_proxied_udp should be able to gather some kind of proxied udp candidates. See  http://crbug.com/873987  for testing instructions.