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Prior to this CL, MediaStreamTrackMetrics hooked up to streams, tracking
not only all of the streams' tracks (CONNECTED/DISCONNECTED) but also
any tracks that might be added or removed from the stream.

This has not been necessary to do for a long time, since the RTP Media
APIs are track-based, not stream-based, and the legacy
addStream()-streams are shimmed on top of addTrack(). Adding a track to
such a stream causes addTrack(), and there is no need for the
MediaStreamTrackMetrics class to observe the streams.

With this CL, MediaStreamTrackMetrics only care about tracks (direction,
kind and id). This also covers the Unified Plan use case of tracks not
necessarily belonging to any stream.

There is room for improvements in this area, such as counting tracks
added through replaceTrack(), but that should be handled separately.
This CL removes the dependency on streams which unblocks
RTCRtpTransceiver/Unified Plan work.

Bug: 810708, 777617
Change-Id: I347e729560717be83b7b1a3571f25a8497d65e46
Reviewed-on: https://chromium-review.googlesource.com/1117691Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572521}