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This adds to the SiteEngagement UKM collection to track the final
engagement score, rounded down to the nearest multiple of 10 to reduce
the granularity of the collection.

Additionally adds a new metrics utility function to perform linear
bucketing (similar to the existing exponential bucketing function).

Bug: 803544
Change-Id: I1c3031e94d4cce28995ab53095c6a4df3e39f65e
Reviewed-on: https://chromium-review.googlesource.com/939876
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540943}