layout delay meter: use own viewport-observer

[samouri]

summary
Teach the LayoutDelayMeter to use its own ViewportObserver instead of relying on Resources to call enterViewport()

Partial for #30620

[dvoytenko]

One thing that's missing is the unobserve when the element is disconnected. Overall the modal of the layout-delay-meter is super similar to the loading indicator. But I refactored it slightly different. And IMHO that looked a bit better. In that model this would look like this:

1. There's a single "instance" per window that deals with layout-delay-meter for any element in that window. Let's call it LoadDelayTracker.
2. CE only needs to call track(this) and untrack(this) on the LoadDelayTracker.
3. The LoadDelayTracker would instantiate a single instance of InOb for its window.
4. The LoadDelayTracker would keep some minimal per-element state in a WeakMap.
5. That's it.

[samouri]

Great idea. Its a bigger shift away in code style / a bigger refactor, but I agree it'd be much cleaner! Luckily we can just remove it outright :)