Add testing config for Release notes notification experiment.

Bug: b/156308432
Change-Id: I8634b4dcffe616f9ce950a6d9500e957f4e83867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2400587
Commit-Queue: Rachel Carpenter <carpenterr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805606}

testing/variations/fieldtrial_testing_config.json

@@ -5430,6 +5430,21 @@
             ]
         }
     ],
+    "ReleaseNotesNotification": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ReleaseNotesNotification"
+                    ]
+                }
+            ]
+        }
+    ],
     "RenderDocument": [
         {
             "platforms": [