Bug 1560924 [wpt PR 17423] - Update interfaces/webrtc.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/webrtc.idl (#17423)

Source: https://github.com/tidoust/reffy-reports/blob/7e64fc0/whatwg/idl/webrtc.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/548532003
--

wpt-commits: 4501cf73a612984558cb2babbd4287f149ab2591
wpt-pr: 17423

testing/web-platform/tests/interfaces/webrtc.idl

@@ -466,6 +466,7 @@ enum RTCIceTransportState {
 };
 
 enum RTCIceRole {
+    "unknown",
     "controlling",
     "controlled"
 };