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This has been running only with sparse histograms for some time now and
has been helpful in finding metrics that overflow.  Now extend this to
check the remaining bulk of histograms.

While the original CL looked for many different reasons of acquiring a
negative sample count, it's now known that there is only a single
reason and so that is the only reason checked for here.

Bug: 682680
Change-Id: I96785436468fdbe34f4221caf7a79da299edeca2
Reviewed-on: https://chromium-review.googlesource.com/741004Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513059}