[Track Stats API] Support all types of MediaStreamTrack about mediacapture-extensions

[henbos]

One possibility discussed, but without consensus from editors present, was to define metrics such that they could be implemented, but whenever not implemented for a particular source, you'd just return empty dictionary (somewhat related issue about feature detection: #101)

[henbos]

An implementation update here on the Chromium side:

While all types of tracks share the same stats logic for delivered and discarded frames (which happen in the renderer), and the same logic for drops triggered around the VideoTrackAdapter parts of the code, there is one kind of frame drop that needs additional implementation effort on a per-source basis.

And that is if the frame was dropped by the source prior to reaching the renderer process. For example if the capturer process runs out of buffers, that's a dropped frame that needs to be signalled. E.g. it's one of those rare circumstances that totalFrames increments but neither deliveredFrames or discardedFrames do.

So it's not true that there is no additional implementation effort in supporting new sources, but it is only needed in cases where drops can happen by the source.

[henbos]

I'm not convinced that sources that are not backed by a real-time device is able to drop frames. For example a canvas based track would only generate frames in a pace that it is able to achieve, so there would be no need to drop the frame before delivery, and thus no extra implementation effort regarding keeping track of dropped frames for these types of sources.

That's just a theory though