webrtcbin: Chain up to parent constructed method

Failing to do so makes GstWebRTCBin invisible to the leaks tracer.

Part-of: <gstreamer/gstreamer!1587>
29 jobs for !2570 with backport-mr-1587-into-1.18-webrtcbin-leaks-tracer in 1 minute and 25 seconds (queued for 5 seconds)
merge request