Prevent site engagement scores from decaying when Chrome isn't in use.

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..a051f9c9b749757b31acc55613d944f4f83457df 100644
--- a/chrome/browser/engagement/site_engagement_score.h
+++ b/chrome/browser/engagement/site_engagement_score.h
@@ -63,6 +63,13 @@ class SiteEngagementScore {
     MEDIUM_ENGAGEMENT_BOUNDARY,
     HIGH_ENGAGEMENT_BOUNDARY,
 
+    // If this many hours passes with no engagement of any kind, the next
+    // points-adding event (or browser startup) will reset each score's last
+    // engagement time to prevent decay when Chrome isn't in use. Times are set
+    // to (now - (overall_last_engagement_time - score_last_engagement_time)).
+    // This value MUST be at least one second greater than the decay period.
+    OBSOLETE_LAST_ENGAGEMENT_PERIOD_IN_HOURS,

[calamity, 2016/06/23 05:18:40]

This name is weird. It makes it look like this is an obsolete variation.

[dominickn, 2016/06/27 02:53:34]

Done.

+
     MAX_VARIATION
   };
 
@@ -81,6 +88,7 @@ class SiteEngagementScore {
   static double GetBootstrapPoints();
   static double GetMediumEngagementBoundary();
   static double GetHighEngagementBoundary();
+  static double GetObsoleteLastEngagementPeriodInHours();
 
   // Update the default engagement settings via variations.
   static void UpdateFromVariations(const char* param_name);
@@ -109,10 +117,7 @@ class SiteEngagementScore {
   bool MaxPointsPerDayAdded() const;
 
   // Resets the score to |points| and resets the daily point limit. If
-  // |updated_time| is non-null, sets the last engagement time and last
-  // shortcut launch time (if it is non-null) to |updated_time|. Otherwise, last
-  // engagement time is set to the current time and last shortcut launch time is
-  // left unchanged.
+  // |updated_time| is non-null, sets the last engagement time to that value.
   void Reset(double points, const base::Time updated_time);
 
   // Get/set the last time this origin was launched from an installed shortcut.
@@ -123,6 +128,14 @@ class SiteEngagementScore {
     last_shortcut_launch_time_ = time;
   }
 
+  // Get/set the last time this origin recorded an engagement change.
+  base::Time last_engagement_time() const {
+    return last_engagement_time_;
+  }
+  void set_last_engagement_time(const base::Time& time) {
+    last_engagement_time_ = time;
+  }
+
  private:
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PartiallyEmptyDictionary);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PopulatedDictionary);