Milestone timing compatibility with CrUX data

Lighthouse sometimes reports different data from what would be collected in CrUX:

- CrUX would use `largestContentfulPaintAllFrames` for LCP, but the Lighthouse report currently doesn't surface that value
- CrUX counts redirect time as part of LCP, but with Lighthouse I get the same LCP for youtube.com and m.youtube.com

(I'm a bit fuzzy on how it works exactly, so let me know if I'm getting something wrong here.)

I was wondering if it would make sense to bring these more in line with each other.