Bug 1601034 - Add a ProcessedMediaTrack layer in MediaPipelineTransmit to handle replaceTrack of ended tracks. r=dminor,padenot

Before this patch, if a send audio MediaStreamTrack ended, we ended up not
sending anything over the network. If replaceTrack() at that point replaced the
ended track with a live one, we'd start sending data again, but the rtp stream
would continue from where the previous track ended.

Having a gap in audio like that would confuse a receiver's *video* jitter
buffer, because it's trying to sync to an audio track that just had a massive
amount of "jitter" (it can't tell the difference).

This patch fixes this by adding a track layer in MediaPipelineTransmit that
remains active for as long as the MediaPipeline is active. Thus if the send
audio MediaStreamTrack ends, we continue sending silence over the network, which
the receiver can understand. If later replaced, the receiver sees real audio
instead of silence and continues gracefully.

Differential Revision: https://phabricator.services.mozilla.com/D56619

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extra : moz-landing-system : lando