Add WebRTC Stats spec

[trptcolin]

This proposes bringing in the WebRTC Stats spec. The driver here is updating the RTCStatsType enum values, as described in microsoft/TypeScript#36650, but there are other changes that the README-recommended fetch-idl process brings in as well.

My initial thinking was that getting all the spec updates in is preferable, but I'm also not totally sure where browser adoption is for this spec, so I'm not sure what level belongs in TypeScript. Could isolate to just RTCStatsType to mitigate risk.

Side note: importing this spec via the README instructions did require a bit of editing to remove the text "WebIDL" in front of each WebIDL block, due to a .idlHeader element with that text appearing inside the .idl code block. It looks like a number of other specs have a similar thing going on:

• Gamepad
• Push
• Screen Orientation
• Selection
• Web Share
• and this one, WebRTC Stats

I'm happy to open a separate issue to suggest some kind of filtering to avoid similar manual IDL-editing effort in the future if that makes sense - just let me know.

[ghost]

All CLA requirements met.

[saschanaz]

I guess it's relatively safer to add experimental dictionary fields, as it won't cause "undefined is not a function" error.

For .idlHeader issue this should work: 97c3235

[orta]

We're struggling with getting the adoption info for this also, querying caniuse for some of the identifiers in here either just in Firefox or just in Safari - IMO, if you constrain it to just RTCStatsType we'll merge but until then we'll probably need to see some larger evidence of use 👍🏻

[saschanaz]

Using Searchfox and webkit-search can be helpful for that.

• Firefox: https://searchfox.org/mozilla-central/source/dom/webidl/RTCStatsReport.webidl
• Safari: https://webkit-search.igalia.com/webkit/source/Source/WebCore/Modules/mediastream/RTCStatsReport.idl

I can see at least some fields are supported in Firefox, but not all.

[orta]

Thanks @saschanaz - those two sites are really useful (I've added them to the README so we can use it more often) - I think on the whole most of these new fields are in FF/Safari which means it's reasonable to merge 👍🏻