PSA: Extra usages of video-content-type RTP extension are gone

[Harald Alvestrand]

The video-content-type RTP header extension, which had been documented to carry one bit only (screenshare or not screenshare) had been used to carry some other information in the past.

In WebRTC, this was purely used for some no-longer-needed statistics.

This usage, and the associated APIs, has now been removed.

See https://crbug.com/webrtc/15383 for details.

Harald Alvestrand