Tracking bug for adding UKM reporting for First Scroll Delay.

UKM already gets reports for
 - Event.ScrollBegin.Wheel
 - Event.ScrollUpdate.Wheel
 - Event.ScrollBegin.Touch
 - Event.ScrollUpdate.Touch
which record scrolling latencies for the initial and subsequent events associated with wheel and touch driven scrolling.

These collections don't distinguish which scrolling interaction these latencies are measuring. That is, measurements of the first scrolling interaction (performed, perhaps, while page load and initialization code is consuming resources) are reported the same way as measurements during idle periods.

We want to measure latencies of first-scroll interactions as a special case so that we can detect problems and verify fixes to first-scroll performance.