Add SiteEngagement experiment to field trial config.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 381 matching lines @@
             "params": {
                 "dismissal_threshold": "1"
             }
         }
     ],
     "SimpleCacheTrial": [
         {
             "group_name": "ExperimentYes"
         }
     ],
+    "SiteEngagement": [
+        {
+            "group_name": "AggressiveAccumulation",
+            "params": {
+                "bootstrap_points": "24",
+                "decay_period_in_hours": "2",
+                "decay_points": "0",
+                "decay_proportion": "0.984",
+                "first_daily_engagement_points": "1.5",
+                "hidden_media_playing_points": "0.01",
+                "high_engagement_boundary": "50",
+                "last_engagement_grace_period_in_hours": "1",
+                "max_decays_per_score": "4",
+                "max_points_per_day": "15",
+                "medium_engagement_boundary": "15",
+                "navigation_points": "1.5",
+                "score_cleanup_threshold": "0.5",
+                "user_input_points": "0.6",
+                "visible_media_playing_points": "0.06",
+                "web_app_installed_points": "5"
+            }
+        }
+    ],
     "SpeculativeLaunchServiceWorker": [
         {
             "enable_features": [
                 "SpeculativeLaunchServiceWorker"
             ],
             "group_name": "Enabled"
         },
         {
             "group_name": "Disabled"
         }
@@ skipping 98 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }