[webrtc-pc] transceiver.direction isn't initialized.

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== transceiver.direction isn't initialized. ==
While the options dictionary [RTCRtpTransceiverInit](https://w3c.github.io/webrtc-pc/#idl-def-rtcrtptransceiverinit) has `direction = "sendrecv"`, it is unused.

Also, we need to initialize transceiver.direction not just in [addTransceiver](https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-addtransceiver), but in the [Create a Transceiver](https://w3c.github.io/webrtc-pc/#dfn-create-an-rtcrtptransceiver) algorithm, used in several places.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1206 using your GitHub account