Add Media Capture and Streams Extensions

[foolip]

No description provided.

[dontcallmedom]

@foolip I've resisted adding mediacapture-extensions because of its unclear status: it describes itself as unofficial, and is a hodge-podge of ideas of different maturity and level of support.

[Elchi3]

The MediaStreamTrackVideoStats interface apparently ships in Chrome and we would have detected this automatically in the https://github.com/openwebdocs/mdn-bcd-collector project if the IDL for it would have been in webref. I guess the reason it is not in webref is because browser-specs doesn't include the spec it's defined in (this PR).

Just adding this as an FYI and subscribing myself to this PR to know if it gets merged or closed (in case this will get closed unmerged we will probably add the IDL definitions to our custom IDL collection).

[dontcallmedom]

my proposal to the WebRTC WG to address this case w3c/mediacapture-extensions#132