Add PlayoutStatistics API

[Hernqvist]

The PlayoutStatistics API for WebAudio (previously incubated in WICG) was discussed in the Audio WG meeting on august 19.

It was agreed that we should make a PR to add the API to WebAudio, with some changes:

• The type of the latency properties should be changed to a duration, not a timestamp.
• The "fallback"-terminology should be changed to something more aligned with industry standards, possibly "underrun".

[padenot]

I was reading definitions about DOMHighResTimeStamp and I think we should just use double. It can be a duration, but there are fairly strong normative requirements that we don't need. A number of seconds or milliseconds is simple to reason about (especially for audio people).

As discussed during the call, I agree that underrun is a more common term.

https://w3c.github.io/hr-time/#moments-and-durations