Previously WebrtcTransport was using webrtc::FakeAudioModule, but it
doesn't work properly when the client tries to read data from audio
tracks. Replaced it with the new remoting::WebrtcAudioModule class.

BUG=638505

Review-Url: https://codereview.chromium.org/2394433003
Cr-Commit-Position: refs/heads/master@{#423324}