GitLab

Adds unit tests that simulate tab activity and test the UKMs logged by
TabMetricsLogger.

Bug: 784639
Change-Id: Id5854b3e2b38b98ecc3d654ac93a24578e26ae3d
Reviewed-on: https://chromium-review.googlesource.com/791868
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520604}