The UMA.SamplingRatePerMille sparse histogram is being incremented so frequently that it sometimes overflows the 31-bit sample count and rolls negative.  Desktop does uploads about every 30 minutes (and resets the counters) so whatever extension these are, it's happening on the order of 1M times per second on some occasions.

https://uma.googleplex.com/p/chrome/histograms/?endDate=latest&dayCount=7&histograms=UMA.NegativeSamples.Histogram%2CUMA.NegativeSamples.Reason%2CUMA.SamplingRatePerMille&fixupData=true&showMax=true&filters=isofficial%2Ceq%2CTrue&implicitFilters=isofficial

Given that this is a "slow" sparse histogram, which involves acquiring a lock, there may be significant overhead here, hence the P2.