Our two ContextualSearchDuration[Non]Prefetched histograms show some suspicious numbers, and I've recently seen an assert fire that seems to indicate a logic flaw.  I think we should change the assert to an early return, which should cleanup those histogram numbers.

https://uma.googleplex.com/p/chrome/histograms?endDate=20170131&dayCount=1&histograms=Search.ContextualSearchDurationNonPrefetched%2CSearch.ContextualSearchDurationPrefetched&fixupData=true&showMax=true&filters=channel%2Ceq%2C4%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial