Implement an observer interface for the site engagement service.

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 7bebca721288261319567a1b77d8b93ecc64c414..a09dc6791af222722c9b1529eb47fea331ed0bcb 100644
--- a/chrome/browser/engagement/site_engagement_score.h
+++ b/chrome/browser/engagement/site_engagement_score.h
@@ -16,8 +16,10 @@ class Clock;
 
 class SiteEngagementScore {
  public:
-  // The parameters which can be varied via field trial. All "points" values
-  // should be appended to the end of the enum prior to MAX_VARIATION.
+  // The parameters which can be varied via field trial.
+  // Any new point value that increases engagement based on direct user activity
+  // should be added after POINTS_INCREMENT_FIRST and before
+  // POINTS_INCREMENT_LAST.
   enum Variation {
     // The maximum number of points that can be accrued in one day.
     MAX_POINTS_PER_DAY = 0,
@@ -30,6 +32,7 @@ class SiteEngagementScore {
 
     // The number of points given for navigations.
     NAVIGATION_POINTS,
+    POINTS_INCREMENT_FIRST = NAVIGATION_POINTS,

[benwells, 2016/06/01 01:44:45]

Is it just me, or will it be awkward now to add more variations which increment
points?

Update: I just saw calamity's comments and can see this has already been
addressed... but I'm not convinced that inserting values into this enum down the
track will be a good idea. Seems like there is a potential for error there.

You should at least mention in a comment that if anything is inserted field
trials should be updated to target specific versions, but even that feels
fraught to me.

[dominickn, 2016/06/01 02:54:13]

Acknowledged. I'll remove this.

 
     // The number of points given for user input.
     USER_INPUT_POINTS,
@@ -40,6 +43,11 @@ class SiteEngagementScore {
     VISIBLE_MEDIA_POINTS,
     HIDDEN_MEDIA_POINTS,
 
+    // Any new points value that increases engagement directly based on user
+    // input should be inserted above this, and update this enum value
+    // appropriately.
+    POINTS_INCREMENT_LAST = HIDDEN_MEDIA_POINTS,
+
     // The number of points added to engagement when a site is launched from
     // homescreen or added as a bookmark app. This bonus will apply for ten days
     // following a launch; each new launch resets the ten days.
@@ -76,6 +84,7 @@ class SiteEngagementScore {
   static double GetBootstrapPoints();
   static double GetMediumEngagementBoundary();
   static double GetHighEngagementBoundary();
+  static double GetMinimumEngagementIncrement();
 
   // Update the default engagement settings via variations.
   static void UpdateFromVariations(const char* param_name);