Minimal event timing proposal

Based on our discussion on First Input Delay
<https://docs.google.com/document/d/1Tnobrn4I8ObzreIztfah_BYnDkbx3_ZfJV5gj2nrYnY/edit>
at
the last WG meeting, I've put together a minimal proposal
<https://docs.google.com/document/d/10CdRCrUQzQF1sk8uHmhEPG7F_jcZ2S3l9Zm40lp3qYk/edit#heading=h.fbdd8nwxr7v4>
for
an event timing API.

The extensions to the DOM spec are fairly straight forward, and the API
itself is pretty bare bones. The main question is whether or not
dispatching an entry per DOM event is too expensive.

If it is, we'll need to devise a method to only report a subset of events.

I'd appreciate any feedback you have,
Tim