GitLab

Restore tests for single-sample storage in histograms.

These tests were removed from the main CL because they were
believed to be the cause of some instability in other tests.
Without the tests, everything seems fine so now it's time to
restore these.

Main CL: https://codereview.chromium.org/2811713003/

BUG=705342

Review-Url: https://codereview.chromium.org/2850233002
Cr-Commit-Position: refs/heads/master@{#468396}