LGTM. We probably want to add the {first,last}VisibleTimes to ads/google/a4a/google-data-reporter.js#googleLifecycleReporterFactory. Not sure what the best parameter to use for that would be -- probably a met.AD_SLOT_NAMESPACE of some form. Maybe Beng or Warren can comment?

One small suggestion: Would it make more sense to call it mostRecentVisibleTime instead of lastVisibleTime? When I first read it, I took lastVisible to mean something like "last time before the page is destroyed", rather than "most recent time that page became visible".

This seems like a good suggestion and I was going to do it. But I found bunch of similarly named properties with that exact meaning named "last visible time". So, with that, I'd rather keep it as is. But thanks a lot for suggestion.

@tdrl Two changes since last review:

1. I pass met.a4a query parameters in the reporter.
2. I also signal preAdThrottle. I'll be happy to rename if desired. But I wanted to have a directly comparable (and hopefully same name) signal as in 3p that directly pings the layoutCallback start.

PTAL.