Typescript: Include WebRTC data channel stuff

[TomasHubelbauer]

Please see Microsoft/TypeScript#23245 for context. This needs to update src\lib\dom.generated.d.ts so I'm opening a PR here instead of in the TypeScript repository. I will be working on this, but can't guarantee any deadlines/estimates, so anyone else is welcome to race me to PRing this.

[a-tarasyuk]

@TomasHubelbauer I've created PR. @mhegazy could you review it?

[mhegazy]

Looks like a duplicate of https://github.com/Microsoft/TypeScript/issues/23245 and should be fixed by https://github.com/Microsoft/TSJS-lib-generator/pull/424

[TomasHubelbauer]

I'm such a goof... I thought I opened this in the TSJS-lib-generator repo :-(