webrtcbin: Chain up to parent constructed method

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

Part-of: <gstreamer/gst-plugins-bad!2570>
29 jobs for !2570 with backport-mr-1587-into-1.18-webrtcbin-leaks-tracer in 62 minutes and 23 seconds (queued for 18 seconds)
merge request