GitLab

The threshold is controlled using field trial.
Also, add few more histograms to track any regressions.

Change-Id: I7901f70196dd23de5eec6fc9574602c71fbe0235
Bug: 882913
Reviewed-on: https://chromium-review.googlesource.com/1214748
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590550}