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There are two reasons for this change:

1. The current code path doesn't work correctly if the media stream and the output devices clocks don't match up *exactly*. Underruns/lipsync issues can occur.

2. This allows for audio to be explicitly buffered for some period of time, which I plan to utilize in the cast_streaming receiver code.

Currently this code path is not often used, since it only happens when you get a media stream from getUserMedia and plug it into a media player without going through webrtc.

Review URL: https://codereview.chromium.org/856843002

Cr-Commit-Position: refs/heads/master@{#313193}