Add fieldtrial testing config for Heavy Ads

Adds an enabled expirement for the Heavy Ads Intervention.

Change-Id: Icf8f645fb171f2d97568915bae595e23910dc5dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300262Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788736}

testing/variations/fieldtrial_testing_config.json

@@ -3239,6 +3239,25 @@
             ]
         }
     ],
+    "HeavyAdIntervention": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "HeavyAdIntervention"
+                    ]
+                }
+            ]
+        }
+    ],
     "HideAutofillSuggestions": [
         {
             "platforms": [