dillon1000/react

Track event times per lane on the root (#19387)

* Pass event time to markRootUpdated

Some minor rearranging so that eventTime gets threaded through. No
change in behavior.

* Track event times per lane on the root

Previous strategy was to store the event time on the update object
and accumulate the most recent one during the render phase.

Among other advantages, by tracking them on the root, we can read the
event time before the render phase has finished.

I haven't removed the `eventTime` field from the update object yet,
because it's still used to compute the timeout. Tracking the timeout
on the root is my next step.