Implemented further mixer interface change suggestions from https://codereview.webrtc.org/2386383003/

Changed mixability status into AddSource/RemoveSource. Added 'ssrc()'
method to the MixerSource interface. Removed unnecessary member 'num_audio_sources_' and made the mixer be refcounted.

BUG=webrtc:6346
NOTRY=True

Review-Url: https://codereview.webrtc.org/2408683002
Cr-Original-Commit-Position: refs/heads/master@{#14612}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 116ec6da50c7c82b2341b97e2d6d512ade3f6e00
5 files changed