iOS NativeDemo: media_session woes

- Repo version: dfa755258a93135732d74c448828eb4438a8d33f
- Signal server: `#define kServerURL @"http://demo.openwebrtc.org"`

Calling web-to-ios yields "Failed to find `media_session` for candidate" and connection is never established. Full console log: https://gist.github.com/nevyn/3fcd29751d44e6a45d66da199b17d46f

Calling ios-to-web yields a crash: https://gist.github.com/nevyn/224f863e9761e1d3794407d0904d39b1

Calling ios-to-ios yields same error as web-to-ios (Failed to find media_session).

They all seem related to some kind of "media session" problem, so... that might be helpful.