Proposal: add "group" concept to Performance Timeline

I'd like to propose that we add "group" concept to Performance Timeline:
https://github.com/w3c/performance-timeline/pull/9

- This is necessary to address the multi-request case in Nav+Resource
Timing (see [1])
- This allows us to refactor Frame Timing to use "group" instead of
sourceFrameNumber

... and I believe that it's a generally useful and powerful concept that
enables Performance Timeline to communicate event cascades.

Thoughts?

ig

[1] https://lists.w3.org/Archives/Public/public-web-perf/2015Feb/0059.html