*Contant emails:*
h...@chromium.org, h...@chromium.org

*Explainer:*
This is a small addition to the MediaStreamTrack 
<https://rawgit.com/w3c/mediacapture-main/master/getusermedia.html#dom-mediastreamtrack>.
 
The new `track.stats` attribute getter allows a JS app to obtain statistics 
of a track, which in the case of video refers to frame counters. For 
example, if you use getUserMedia() to open a camera at a specific frame 
rate setting, this API lets you know how many frames the camera is actually 
producing (e.g. frame rate may be lower in low light conditions or higher 
than the app wants due to HW capabilities) and how many frames, if any, are 
discarded or dropped by the user agent, which is useful for investigations 
and experiments (some concrete examples mentioned here 
<https://github.com/mozilla/standards-positions/issues/895>).

*Spec:*
https://w3c.github.io/mediacapture-extensions/#mediastreamtrack-statistics
This is an extension spec, owned by the same working group who owns the 
main spec.

The spec and implementation currently only covers video frames, but similar 
audio frame counters are being considered for the future as well.

*Summary:*
Add frame counters to MediaStreamTrack.

*Is this feature supported on all six Blink platforms?*
Yes.

*Measurements*
Will add [Measure].

*Risks*
Compatibility: MediaStreamTrack is supported on all browsers and the new 
stats API is a read-only statistics API rather than core functionality, so 
interop risks should be relatively limited.

Signals from other implementers:
- Firefox: positive (request for position 
<https://github.com/mozilla/standards-positions/issues/895>)
- Safari: no signal at the time of writing (request for position 
<https://github.com/WebKit/standards-positions/issues/260>)

Ergonomics: Used together with MediaStreamTrack and WebRTC 
(RTCPeerConnection)

Activation: N/A

*Web Platform Tests*
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/external/wpt/mediacapture-extensions/MediaStreamTrack-video-stats.https.html

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