The example that brought up this issue was:
1. Do offer/answer exchange.
2. Later, remove the audio/video stream.
3. Add back a new stream, that contains only the audio track.
4. Do new offer/answer.
The new offer didn't have the new stream ID, but code elsewhere was
expecting one. As a result, the send stream is never hooked up to the
audio track, and audio packets aren't sent.
BUG=chromium:611708
Review-Url: https://codereview.webrtc.org/2810733003
Cr-Commit-Position: refs/heads/master@{#17709}
2f425aa6b5