Don't break back-compat of the LHR

[paulirish]

We'll have people saving lhr.json files more often now. And if they drag one from folder into devtools audits, they'll expect to see a report.

My proposal to make this easier:

• the LHR gets a .config property which is the actual config.
• do the generateReportJson()/etc work inside of the generateHTML method
• optional: we could even expose a more static scoreReport method in case other modules wanted scores/certifications. (Similar public API-ness makes sense for the pw-metrics-events metric gathering_

But to paraphrase pavel, the sooner you start guaranteeing the backcompat of this API the better.

[patrickhulce]

dupe of #2668