send Hero Element (TopSites) painted telemetry

[dmose]

This will (approximately) be adding top_sites_painted_ts to the session telemetry.

Note that doing it will be a bit fun. My suspicion is that the right way to do it is to look at the technique described in http://stackoverflow.com/a/34999925, but to use two requestAnimationFrame's as suggested by #2005 (comment) since rAF is less likely to jank than setTimeout.

[dmose]

As of this writing, the exact data schema is still being sorted out in https://github.com/mozilla/activity-stream/pull/2660/files.

[tspurway]

This is a measure of the time to when TopSites are available for interaction

[dmose]

There's a user story for the dashboarding of this in #2838.

[dmose]

I've filed #2965 to track the performance of filling in the screenshots for the times when a tab needs that.