Add support for a signaling thread message loop to WebRtcAudioRenderer.
This CL adds the concept of a signaling thread to WebRtcAudioRenderer.
The initial implementation of PeerConnectionDependencyFactory::GetWebRtcSignalingThread
returns the main render thread (which is currently being used as the signaling thread).
In an upcoming patch, GetWebRtcSignalingThread will start returning a different message loop proxy.
BUG=369796
Review URL: https://codereview.chromium.org/
687533003
Cr-Commit-Position: refs/heads/master@{#301811}