Added `RestartLocalTracks` and `RestartRemoteTracks` methods to
`MediaStreamHandlerContainer` which will redo the track handlers'
initial setup; most importantly, this will re-connect the
renderer/capturer/etc. to a channel which was destroyed and then
re-created.
Also added `AcceptRemoteTracks` method to MediaStreamSignaling, which
does the inverse of `RejectRemoteTracks`. Effectively this will notify
sinks that the track is live again, after previously being set to
`kEnded` when it was rejected.
BUG=webrtc:2136
Review URL: https://codereview.webrtc.org/1231613002
Cr-Commit-Position: refs/heads/master@{#9600}
be37888b6d