safari: fix trackevent transceiver property
safari: fix trackevent transceiver property

Fixes #984.

Test Plan:
- verify that 'transceiver' in RTCTrackEvent.prototype works in current safari
- call SRD with a SDP that will trigger ontrack
- inspect track event, look at transceiver property

Acidburn0zzz/adapter,webrtc/adapter,webrtc/adapter,Acidburn0zzz/adapter,Acidburn0zzz/adapter