Discard link to media channel when audio sender stopped.
Discard link to media channel when audio sender stopped.

Bug: chromium:822799
Change-Id: Ib863cf048318b04369cc51ed1b1c8b03010a2fd2
Reviewed-on: https://webrtc-review.googlesource.com/62941
Commit-Queue: Harald Alvestrand <ce816cd73d80f958d82de1df356058c2d2bca378@webrtc.org>
Reviewed-by: Steve Anton <93f73658da16553b94c040887a439a155812e54e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22503}
ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc