GitLab

Sparse histograms don't have a single "values vector" that
can be created during construction.  Rather, they create a
list of value "records" that can be easily added to as needed.

BUG=546019
TBR=mark
mark: gn and gyp (new files)

Review URL: https://codereview.chromium.org/1734033003

Cr-Commit-Position: refs/heads/master@{#381699}