Perf Metrics & Scoring vNext

[paulirish]

Some small changes we plan on making to the metrics we expose and how we score them

• Add FCP to the LH results Add FCP to the LH results #3761
• Metrics: rename "First Interactive" and "Time To Consistently Interactive" Metrics: rename "First Interactive" and "Time To Consistently Interactive" #3655
• Redo the weighting
• TTI moved to depending on FCP. we should follow
• Network Idle was adjusted. We can also filter out non-GET and Failed requests

proposal doc: LH Performance Score 2018Q1 Revision

cc #4333

[vinamratasingal-zz]

Based on feedback from the LH scoring committee meeting, we have an overall preliminary LGTM but a) need to dig into the feedback a bit more and b) need to see how the new score shapes up on a few websites and dig into the traces a bit more. We're hoping to have this ready before the next scoring committee meeting in late March.