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

chrome/browser/engagement/site_engagement_service.h

Index: chrome/browser/engagement/site_engagement_service.h
diff --git a/chrome/browser/engagement/site_engagement_service.h b/chrome/browser/engagement/site_engagement_service.h
index e18e9808a9fbda9d7871e3a90cb7d759a50293f1..a98dd6c46a02cbecb64f4b68742238477822f6db 100644
--- a/chrome/browser/engagement/site_engagement_service.h
+++ b/chrome/browser/engagement/site_engagement_service.h
@@ -126,6 +126,8 @@ class SiteEngagementService : public KeyedService,
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, IsBootstrapped);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, EngagementLevel);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, ScoreDecayHistograms);
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest,
+                           PersistLastEngagementTime);
   FRIEND_TEST_ALL_PREFIXES(AppBannerSettingsHelperTest, SiteEngagementTrigger);
 
   // Only used in tests.
@@ -139,10 +141,21 @@ class SiteEngagementService : public KeyedService,
   SiteEngagementScore CreateEngagementScore(const GURL& origin);
   const SiteEngagementScore CreateEngagementScore(const GURL& origin) const;
 
-  // Post startup tasks: cleaning up origins which have decayed to 0, and
-  // logging UMA statistics.
+  // Runs site engagement maintenance tasks.
   void AfterStartupTask();
-  void CleanupEngagementScores();
+
+  // Removes any origins which have decayed to 0 engagement. If
+  // |update_last_engagement_time| is true, the last engagement time of all
+  // origins is reset by calculating the delta between the last engagement event
+  // recorded by the site engagement service and the origin. The origin's last
+  // engagement time is then set to clock_->Now() - delta.
+  //
+  // If a user does not use the browser at all for some period of time,
+  // engagement is not decayed, and the state is restored equivalent to how they
+  // left it once they return.
+  void CleanupEngagementScores(bool update_last_engagement_time);
+
+  // Records UMA metrics.
   void RecordMetrics();
 
   // Returns the median engagement score of all recorded origins.
@@ -163,6 +176,12 @@ class SiteEngagementService : public KeyedService,
   void HandleUserInput(content::WebContents* web_contents,
                        SiteEngagementMetrics::EngagementType type);
 
+  // Returns true if the last engagement increasing event seen by the site
+  // engagement service was sufficiently long ago that we need to reset all
+  // scores to be relative to now. This ensures that users who do not use Chrome
+  // at all for an extended period of time do not have their engagement decay.
+  bool IsLastEngagementObsolete();
+
   // Overridden from history::HistoryServiceObserver:
   void OnURLsDeleted(history::HistoryService* history_service,
                      bool all_history,
@@ -195,6 +214,11 @@ class SiteEngagementService : public KeyedService,
   // upload.
   base::Time last_metrics_time_;
 
+  // Keep track of the last time any engagement was recorded. If this date is
+  // too far in the past, assume that the user has not been using Chrome, and
+  // ensure that scores are not decayed as usual.
+  base::Time last_engagement_time_;
+
   base::WeakPtrFactory<SiteEngagementService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementService);