LCP detection to handle similar elements

[DahmaniAdame]

Is your feature request related to a problem? Please describe.
The current LCP detection has a limit related to similar elements.

Sliders case

The LCP element will depend in when the beacon was fired. It can pick up the second or 3rd slider just because of the timing of execution.

Grids case

On products grids, let's say a row with 4 items with the exact same dimensions, we might pick up any of the items. This is also the case for LCP detection with the official Performance API in some occasions.

The ideal behavior is to pick up the first element in these cases to make the outcome predictable and close to what PSI/Lighthouse would detect.

For that to happen, we need to either:

• Feed the beacon with structures to process
• Make the beacon pickup patterns by checking ancestors to figure out if there is a better LCP candidate to consider than the one it managed to pick up

Describe the solution you'd like
N/A

Describe alternatives you've considered
N/A

Additional context
https://secure.helpscout.net/conversation/2715339893/513759/ (grid example)
https://secure.helpscout.net/conversation/2718854167/514542/ (slider example)