Reason for revert: A interface change broke some downstream code in google3. Original issue's description: > Support external audio mixer in webrtc. > > An external audio mixer will be passed from PeerConnectionFactory to > AudioTransportProxy. > > BUG=webrtc:6457 > > Committed: https://crrev.com/f6bcac59e88c3be5ffd73bbb1098f2d82ee316a1 > Cr-Commit-Position: refs/heads/master@{#15556} TBR=solenberg@webrtc.org,aleloi@webrtc.org,deadbeef@webrtc.org # Skipping CQ checks because original CL landed less than 1 days ago. NOPRESUBMIT=true NOTREECHECKS=true NOTRY=true BUG=webrtc:6457 Review-Url: https://codereview.webrtc.org/2562333003 Cr-Commit-Position: refs/heads/master@{#15557}