Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_score.h

Index: chrome/browser/engagement/site_engagement_score.h
diff --git a/chrome/browser/engagement/site_engagement_score.h b/chrome/browser/engagement/site_engagement_score.h
index 33d4a3af5a6fa766fda50cae6071cfac608ce89e..8a1b0a7eb53aa8fb88502a949afc567b89cc6ebc 100644
--- a/chrome/browser/engagement/site_engagement_score.h
+++ b/chrome/browser/engagement/site_engagement_score.h
@@ -21,8 +21,7 @@ class HostContentSettingsMap;
 
 class SiteEngagementScore {
  public:
-  // The parameters which can be varied via field trial. All "points" values
-  // should be appended to the end of the enum prior to MAX_VARIATION.
+  // The parameters which can be varied via field trial.
   enum Variation {
     // The maximum number of points that can be accrued in one day.
     MAX_POINTS_PER_DAY = 0,