[iOS][Safety Check] Add fieldtrial for safety check ios

Add fieldtrial entry for safety check on iOS for experimenting in
Beta.

Bug: 1078782
Change-Id: I9be9b7e8f5d3d65edfe9a386f83433d054793247
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498545Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Sean Harrison <harrisonsean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821220}

testing/variations/fieldtrial_testing_config.json

@@ -6310,6 +6310,21 @@
             ]
         }
     ],
+    "SafetyCheckIOS": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SafetyCheckIOS"
+                    ]
+                }
+            ]
+        }
+    ],
     "SaveEditedPDFForm": [
         {
             "platforms": [