Updates to ResourceTiming entries - followup

Hi Web Perf,

In the last Web Perf WG call, we were asked to analyze more thoroughly some
of the alternatives to our current proposal to expose in-flight resource
requests. I've added a section at the end of the document [1]. Feedback is
appreciated, thanks!

[1]
https://docs.google.com/document/d/18J6JJB4HHssqIfeHjb6sQbRrbcduJFRfkset8edt5L4/edit?usp=sharing