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

chrome/browser/engagement/site_engagement_score.cc

Index: chrome/browser/engagement/site_engagement_score.cc
diff --git a/chrome/browser/engagement/site_engagement_score.cc b/chrome/browser/engagement/site_engagement_score.cc
index e55522045d8d960b462824f6449629d9f2623d7e..07ea4e4baa0ebe70e891a6b0b2883b9a387a8b4d 100644
--- a/chrome/browser/engagement/site_engagement_score.cc
+++ b/chrome/browser/engagement/site_engagement_score.cc
@@ -83,6 +83,7 @@ double SiteEngagementScore::param_values[] = {
     8,     // BOOTSTRAP_POINTS
     5,     // MEDIUM_ENGAGEMENT_BOUNDARY
     50,    // HIGH_ENGAGEMENT_BOUNDARY
+    240,   // OBSOLETE_LAST_ENGAGEMENT_PERIOD_IN_HOURS
 };
 
 const char* SiteEngagementScore::kRawScoreKey = "rawScore";
@@ -139,6 +140,10 @@ double SiteEngagementScore::GetHighEngagementBoundary() {
   return param_values[HIGH_ENGAGEMENT_BOUNDARY];
 }
 
+double SiteEngagementScore::GetObsoleteLastEngagementPeriodInHours() {
+  return param_values[OBSOLETE_LAST_ENGAGEMENT_PERIOD_IN_HOURS];
+}
+
 // static
 void SiteEngagementScore::UpdateFromVariations(const char* param_name) {
   double param_vals[MAX_VARIATION];