Remove WebRtcVideoSendStream2::VideoSink inheritance. Remove sending black frame on source removal.

BUG=webrtc:6371,webrtc:6983

Review-Url: https://codereview.webrtc.org/2469993003
Cr-Original-Commit-Position: refs/heads/master@{#16048}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d533aec3b8425edd81c2743f7683c12b74c307b5
4 files changed