Moved the fake input stream's processing onto the audio worker thread.

Moved the fake input stream's processing onto the audio worker thread.

The reason for this is that the fake input stream would not work on Mac
where the audio non-worker code runs on the UI thread. Therefore, the
fake input stream would get starved for instance in the WebRTC audio
quality tests and not play any audio (those tests block the UI thread
while recording).

This patch introduces a worker precisely like the fake audio consumer
used by the fake output stream.

BUG=453907,446859

Review URL: https://codereview.chromium.org/922663002

Cr-Commit-Position: refs/heads/master@{#317557}