Add UMA metrics to the site engagement service.
This CL adds UMA for a number of different statistics, including:
* total cumulative engagement and number of origins
* total origins which reach the capped daily and total engagement scores,
and the percentage of all origins per user which reach the total max
* total number of events which increase the engagement score,
differentiated by type
* engagement scores per origin
* mean and median engagement scores per user over all origins
Most statistics are recorded at non-incognito profile launch, and then
once per hour after that. Engagement type statistics are recorded per
engagement event. Tests are added to verify this behaviour.
BUG=464234
Review URL: https://codereview.chromium.org/1368533004
Cr-Commit-Position: refs/heads/master@{#351973}
10 files changed
