spec for background task throttling

[Ojan Vafai]

At this point we've shipped the initial throttling of timers in background tabs. We're planning on extending that to non-timers, which might change how it all works.

https://github.com/WICG/interventions/issues/34 has Microsoft asking how it works, which implies to me that they're considering implementing this. Seems like a good time to spec what we've shipped so far. 

I think what we'd spec is:

1. The list of APIs that prevent throttling (audio, webrtc, etc).

2. A high-level description of how the budget-based throttling works. I say high-level because we are actively tweaking how it works.

WDYT?