INP attribution helper latestProcessingEnd never initialized

[rviscomi]

latestProcessingEnd is declared without a value, so it's initially undefined.

When comparing the undefined value with entry.processingEnd, the result is always NaN.

And finally, when the NaN value is compared with loaf.startTime the result is always false, so the LoAF entries are always kept.

[brendankenny]

so the LoAF entries are always kept

one bit of good news is that this at least maxes out at MAX_PREVIOUS_FRAMES, so we don't keep more than 50 + 10 (if the 10 longest LoAFs aren't in the most recent 50), but yeah