Remove media sinks from Channel.

Allows removing MediaRecorder which isn't in use apart from channel
unittests, along with it unittests for MediaRecorder that are flaky when
run in parallel can also go.

BUG=
R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1219663008

Cr-Commit-Position: refs/heads/master@{#9558}
8 files changed