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

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index d47493512fa0a027b1bf6d8eb0521224773b5ccd..eb0e6457f51eb97d620ad1b68df8ef9b575fe897 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -73,6 +73,12 @@ const char kSessionExitedCleanly[] = "profile.exited_cleanly";
 // shutdown. Used to determine the exit type the last time the profile was open.
 const char kSessionExitType[] = "profile.exit_type";
 
+// The last time that the site engagement service recorded an engagement event
+// for this profile for any URL. Recorded only during shutdown. Used to prevent
+// the service from decaying engagement when a user does not use Chrome at all
+// for an extended period of time.
+const char kSiteEngagementLastUpdateTime[] = "profile.last_engagement_time";
+
 // An integer pref. Holds one of several values:
 // 0: unused, previously indicated to open the homepage on startup
 // 1: restore the last session.