GitLab

Previously RateCounter was using base::Time instead of base::TimeTicks,
which may cause it to generate incorrect values in some cases. Fxed it
to use TimeTicks. Also updated it to use TickClock interface for tests.

Review-Url: https://codereview.chromium.org/2561173004
Cr-Commit-Position: refs/heads/master@{#437712}