Proposal: add timestamp for when "activation" or "dismissal" starts about navigation-timing

[fergald]

Should there be a deactivationEnd? I think it should be at the basically the same time as unloadEventEnd, which makes it somewhat redundant but for symmetry and for a future where hopefully there is no unload event it would be nice to have.

[nicjansma]

Discussed on the March 13, 2024 W3C WebPerf call: https://docs.google.com/document/d/1k7lB7XWXHBUmxR5YaycYTXWsHVqXelLh0RRDIFnx-gI/edit#heading=h.t4ndg6qhajam

• Noam went over the proposal a bit further, clarifying what it could measure and how it can help identify troublesome page event handlers (on the previous page)
• These types of measurements could be done by a site on its own (transitioning measurements to the next page), but these timestamps would offer a convenience
• Next steps is for RUM vendors to talk to their customers / consumers and see if this is impacting folks that are trying to measure view-transitions, and report back