Allow the site engagement service thresholds to be varied via field trial.

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 e5735605a60285b633ed3707c2a439ed21cd6669..f2e067705f5beadacd265edeb7d067566a82ab79 100644
--- a/chrome/browser/engagement/site_engagement_service.h
+++ b/chrome/browser/engagement/site_engagement_service.h
@@ -25,29 +25,27 @@ class Profile;
 
 class SiteEngagementScore {
  public:
-  // Keys used in the content settings dictionary.
-  static const char* kRawScoreKey;
-  static const char* kPointsAddedTodayKey;
-  static const char* kLastEngagementTimeKey;
-
   // The maximum number of points that are allowed.
   static const double kMaxPoints;
 
   // The maximum number of points that can be accrued in one day.
-  static const double kMaxPointsPerDay;
+  static double g_max_points_per_day;
 
   // The number of points given for navigations.
-  static const double kNavigationPoints;
+  static double g_navigation_points;
 
   // The number of points given for user input (indicating time-on-site).
-  static const double kUserInputPoints;
+  static double g_user_input_points;
 
   // Decaying works by removing a portion of the score periodically. This
   // constant determines how often that happens.
-  static const int kDecayPeriodInDays;
+  static int g_decay_period_in_days;
 
   // How much the score decays after every kDecayPeriodInDays.
-  static const double kDecayPoints;
+  static double g_decay_points;
+
+  // Update the default engagement settings via variations.
+  static void UpdateFromVariations();
 
   // The SiteEngagementService does not take ownership of |clock|. It is the
   // responsibility of the caller to make sure |clock| outlives this
@@ -67,8 +65,15 @@ class SiteEngagementScore {
   bool UpdateScoreDict(base::DictionaryValue* score_dict);
 
  private:
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PartiallyEmptyDictionary);
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementScoreTest, PopulatedDictionary);
   friend class SiteEngagementScoreTest;
 
+  // Keys used in the content settings dictionary.
+  static const char* kRawScoreKey;
+  static const char* kPointsAddedTodayKey;
+  static const char* kLastEngagementTimeKey;
+
   // This version of the constructor is used in unit tests.
   explicit SiteEngagementScore(base::Clock* clock);