Split time to interactive into first_interactive and consistently_interactive

[avadacatavra]

Consistently Interactive:
Find a the first 5 second window W after FMP such that

• W overlaps no tasks longer than 50ms
• For all timestamp t in W, number of resource requests in flight at t is no more than 2.

Now find the last long task L before W.

• Consistently Interactive Candidate is the end of L
• In the case there is no long task before L, Consistently Interactive Candidate = FMP.

Now, Consistently Interactive = max(Consistently Interactive Candidate, DOMContentLoadedEventEnd)

We might need to have some discussion about what definition we want to use for FirstInteractive:

Note (August 28, 2017): There is an equivalent FirstInteractive definition in terms of heavy and light task clusters instead of task envelopes and lonely tasks that leads to a cleaner implementation. See First Interactive - Task Cluster Definition. We have the original definitions in terms of Lonely Tasks below:
Reference: https://docs.google.com/document/d/1GGiI9-7KeY3TPqS3YT271upUVimo-XiL5mwWorDUD4c/preview#

[jdm]

I also just came across https://docs.google.com/document/d/1l50v3ljDeUmj9qBsfdPc3wKe_1CY5KFYI1baye2NhYk/edit?ts=59dfaf65# which doesn't describe two separate metrics.