core(layout-shift-elements): aggregate all remaining elements

[adamraine]

Closes #15423

• Increases the number of trace elements we resolve from 5 to 15
• Aggregates all layout shift elements beyond 15 into an additional table row
• Enables core(trace-elements): remove element score field #15677

[connorjclark]

LGTM

note: this will exacerbate #14583 - CLS is from just the largest shift window, but this audit does not consider windows and shows all shifts. So if you sum these up you'll get something > than CLS (assuming there is more than one shift window).

personally I'd rather show all the shifts regardless of CLS windowing (as the audit does now), and just be OK w/ these not summing to CLS score. the windowing thing for CLS score makes sense in terms of defining a metric, but for fixing layout shifts on a page I feel like it's more important to consider the largest shifts / largest impacted nodes. It won't give the min. path to a smaller CLS but it will have larger user impact imo.

[adamraine]

This test case was moved to computed/metrics/cumulative-layout-shift-test.js. The remaining tests were already covered in that file.