report(perf-audits): adjust presentation of runtime cost audits

[paulirish]

I noticed and fixed some inconsistencies

• main thread work breakdown was always showing in diagnostics, even if total cost was just 40ms.
• bootup time audit was saying "is high" but only when the audit was passing.
• now the failure threshold for the two audits is adjusted.
  • just JS bootup times costing more than 2s, show it. total main thread time costing more than 4s, show that.

[vinamratasingal-zz]

Silly question- are there any UX changes to the report itself? Otherwise, I don't have any input on the logic changes.

[paulirish]

• i'm changing when both audits show up vs get put into "Passed Audits"
• The title of "JavaScript boot-up time is high (> 4s)" (which you see in the report), i'm changing to just "JavaScript boot-up time".

[vinamratasingal-zz]

LGTM