NativeDemo: add missing bit to support JSON messages
NativeDemo: add missing bit to support JSON messages

Other parts of the code already support either using .sessionDescription or
parsing .sdp. This commit adds a check to actually fully support
.sessionDescription in signaling messages.

EricssonResearch/openwebrtc-examples,EricssonResearch/openwebrtc-examples,EricssonResearch/openwebrtc-examples,EricssonResearch/openwebrtc-examples,EricssonResearch/openwebrtc-examples,EricssonResearch/openwebrtc-examples