Record offline histograms with suffix via Histogram::FactoryGet

Also move all offline client namespace constants into one central place.

BUG=616231
TBR=mmenke@chromium.org

Committed: https://crrev.com/8ccbcdc96179ab65038e17cdbddc149e28754830
Cr-Commit-Position: refs/heads/master@{#397233}

Review-Url: https://codereview.chromium.org/2022283003
Cr-Commit-Position: refs/heads/master@{#397623}
14 files changed