GitLab

This adds UMA histograms for keeping track of how long it takes for
the Chrome Cleaner to scan and clean user's machines as well as the
time it takes for users to interact with the Chrome Cleaner prompt
dialog.

Bug: 746987
Change-Id: If4d70c196f68c462834cb0c1e8a82a8bfaa6ea05
Reviewed-on: https://chromium-review.googlesource.com/580087
Commit-Queue: Ali Tofigh <alito@chromium.org>
Reviewed-by: Robert Shield <robertshield@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488475}