Add field testing config for add-supervision dialog

Bug: 943901
Change-Id: I8917e05092856f5f909c8a4e1b60c73ac7e58739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876490Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Dan S <danan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708738}

testing/variations/fieldtrial_testing_config.json

@@ -1560,6 +1560,21 @@
             ]
         }
     ],
+    "ChromeOSParentalControlsSettings": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ChromeOSParentalControlsSettings"
+                    ]
+                }
+            ]
+        }
+    ],
     "ChromeSmartSelection": [
         {
             "platforms": [