Add enabled feature to fieldtrial_testing_config for ClientSideDetectionModel

When adding the fieldtrial_testing_config for this experiment, I missed
the enabled feature field, blocking the rollout to beta.

Change-Id: Ibfe8e0f266c94f76b46881bbb0dcd12a66ad4df1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227395
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774621}

testing/variations/fieldtrial_testing_config.json

@@ -1479,7 +1479,10 @@
                     "name": "Enabled",
                     "params": {
                         "ModelNum": "4"
-                    }
+                    },
+                    "enable_features": [
+                        "ClientSideDetectionModel"
+                    ]
                 }
             ]
         }