The DataUse.Sync.Upload.Bytes sparse histogram is being incremented such that it sometimes overflows the 31-bit sample count and rolls negative.  Desktop does uploads about every 30 minutes (and resets the counters) but because this is incremented by a "byte count" it's very easy to exceed the 2GiB mark.

https://uma.googleplex.com/p/chrome/histograms/?endDate=20171011&dayCount=1&histograms=DataUse.Sync.Upload.Bytes%2CUMA.NegativeSamples.Histogram&fixupData=true&showMax=true&filters=isofficial%2Ceq%2CTrue&implicitFilters=isofficial

This metric was added here:
https://codereview.chromium.org/2130453004