Add audio support in WebrtcConnectionToHost, audio unittest

WebrtcConnectionToHost now supports audio streaming.
Also added a unittests that tests audio streaming, both for ICE and
WebRTC protocols.

BUG=638505

Review-Url: https://codereview.chromium.org/2371323007
Cr-Commit-Position: refs/heads/master@{#424232}
10 files changed