Implement an observer interface for the site engagement service.

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 140cd01b1135e78e3b8eb5d14a60d6bff2f2cdfe..af456dc8410ded7a83e35e9aae6087fcdf077dd4 100644
--- a/chrome/browser/engagement/site_engagement_score.cc
+++ b/chrome/browser/engagement/site_engagement_score.cc
@@ -15,6 +15,10 @@
 
 namespace {
 
+// The smallest amount by which engagement is incremented. Updated if necessary
+// in SiteEngagementScore::UpdateFromVariations.
+double gMinimumEngagementIncrement = 0.01;
+
 // Delta within which to consider scores equal.
 const double kScoreDelta = 0.001;
 
@@ -119,6 +123,10 @@ double SiteEngagementScore::GetHighEngagementBoundary() {
   return param_values[HIGH_ENGAGEMENT_BOUNDARY];
 }
 
+double SiteEngagementScore::GetMinimumEngagementIncrement() {
+  return gMinimumEngagementIncrement;
+}
+
 // static
 void SiteEngagementScore::UpdateFromVariations(const char* param_name) {
   double param_vals[MAX_VARIATION];
@@ -137,9 +145,17 @@ void SiteEngagementScore::UpdateFromVariations(const char* param_name) {
   }
 
   // Once we're sure everything is valid, assign the variation to the param
-  // values array.
-  for (int i = 0; i < MAX_VARIATION; ++i)
+  // values array. Update the minimum engagement increment if it has changed.
+  for (int i = 0; i < MAX_VARIATION; ++i) {
     SiteEngagementScore::param_values[i] = param_vals[i];
+
+    // If the value is one used to increment the site engagement score, update
+    // the minimum engagement increment if necessary.
+    if (i >= POINTS_INCREMENT_FIRST && i <= POINTS_INCREMENT_LAST &&
+        param_vals[i] < gMinimumEngagementIncrement) {
+      gMinimumEngagementIncrement = param_vals[i];
+    }
+  }
 }
 
 SiteEngagementScore::SiteEngagementScore(base::Clock* clock,