de-dupe screenshot urls in LHR

[connorjclark]

As suggested in #14679 by @benschwarz – we could save the raw size of the LHR ~100+ KB if we de-duped data url screenshots.

Not just for screenshot-thumbnails, but also final-screenshot (which after #14679 would be equivalent to the last screenshot thumbnail). If we went that route, we'd want to add a top-level screenshots or dataUrls, etc.

We could go further and drop screenshot-thumbnails and final-screenshot as audits, and just have a top-level item like:

screenshots: {
  dataUrls: string[];
  frames: Array<{index: number, timing: number, timestamp: number}>
};

(many other ways to slice this)

and the report renderer would just grab from that to do its thing

[paulirish]

We have other cases where deduping would be advantageous to JSON size.. like all the URLs... The solution is similar.. Having some big urls array and then everything references a urlIndex.. More or less.

Going in this direction...

• we could apply these changes and then formalize the LH.ReportResult which is.. like the usable version of the LHR. And it'd undo these space saving changes.
• we could instead maintain a lil side library (a la median/representative runs) to change the LHR into a SlimLHR which.. isn't officially supported but ... we could kinda maintain some optimizations as a community.

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however.. right now we're thinking it may be premature to go this route..

in some experiments we've found that compression does a pretty great job on LHRs so.. leaving them brotli'd or zstd'd at rest might be the most cost-effective solution: