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This is to support https://crrev.com/c/792315, which adds a new observer
for logging engagement-type UMA in PWA app windows.

The major interface changes are:
- Renamed OnEngagementIncreased to OnEngagementEvent to indicate that it
  is now for all events that modify engagement, not just increases.
- OnEngagementEvent is now called on launch (so now every place where an
  engagement UMA is logged also corresponds to an OnEngagementEvent).
- OnEngagementEvent now takes an EngagementType enum, indicating what
  caused the engagement event.

Because the engagement type is now exposed through the observer (and can
therefore be used for non-metrics purposes), it has been moved out of
SiteEngagementMetrics into SiteEngagementService.

Also refactored SiteEngagementService to record UMA from the
OnEngagementEvent method, rather than in many places.

Bug: 785661
Change-Id: I352d0363c9ee3a40293e7b99d5b8f6e75c0e999b
Reviewed-on: https://chromium-review.googlesource.com/802661Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521639}