Fix threading issues in the audio pipeline for WebRTC remoting protocol.

1. Updated unittests to use a separate thread for audio encoding and
2. WebrtcAudioSinkAdapter was calling AudioStub on a wrong thread.
3. WebrtcAudioModule was destroying base::Timer on a thread different
   from the one on which it is created.

Cr-Commit-Position: refs/heads/master@{#445566}
5 files changed