webrtc: Change H264 examples to use aggregate-mode=zero-latency for best compatibility

The default changed back to none because it broke existing code.

See gstreamer/gst-plugins-good#749

Part-of: <gstreamer/gst-examples!22>
24 jobs for !22 with rtph264pay-zero-latency in 80 minutes and 26 seconds (queued for 3 seconds)
merge request