Add Compute Pressure API

[Elchi3]

Compat features are tagged in BCD: mdn/browser-compat-data#22687

Not sure if https://chromestatus.com/metrics/feature/timeline/popularity/3900 (PressureObserver_Observe) is the best use counter, cc @kenchris

[foolip]

Not sure if https://chromestatus.com/metrics/feature/timeline/popularity/3900 (PressureObserver_Observe) is the best use counter

The other alternative is https://chromestatus.com/metrics/feature/timeline/popularity/3899 (the constructor) but so far they show the same data. A call to observe() is more "real", but you can go one step further and say we should measure when the callback is actually invoked.

So I would say use the constructor until we see some difference between that and observe().

[foolip]

https://w3c.github.io/compute-pressure/#supported-sources has both "thermals" and "cpu", but only "cpu" is implemented in Chrome. So we should probably consider just CPU pressure the initial feature.

[foolip]

Paging @ddbeck for an opinion on naming. Go with the spec name or match the API name? We already have intersection observer.

[ddbeck]

I looked into this feature a little more. Especially these docs https://developer.chrome.com/docs/web-platform/compute-pressure and this piece in particular:

In this example the "cpu" is the pressure source we are interested in. For now, it is the only
source available. In the future, there may be other sources such as "gpu", "power" or "thermals".

I think what we have here is both generic (the compute pressure API in general) and quite specific (observing CPU pressure specifically). It makes naming and describing this thing a pain. I'm going to review with the understanding that we might get this wrong and need to re-name or describe this later, when its in front of web developers.