Cleanups in WebrtcTransport.
This change includes several changes needed in the Webrtc-based
transport:
1. WebrtcTransportFactory no longer owns the worker thread. Instead
the calling code must provide a thread that has been started already.
This also allows to make WebrtcTransport::Start() synchronous.
2. The direction in which offer and answer is sent is reversed. The
host sends offer to the client and the client replies with an answer.
3. Offer is generated only after OnRenegotiationNeeded() notification is
received.
4. The |is_server| flag in WebrtcDataStreamAdapter is replaced with
|outgoing|, so connections may be created in either direction.
BUG=547158
Review URL: https://codereview.chromium.org/1510333002
Cr-Commit-Position: refs/heads/master@{#364155}
5 files changed
