[iOS][Biometric-Auth] Add experiment config

Bug: 1112783
Change-Id: I3710efd6972d205af7f0750224ce407b6b534235
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377723
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802209}

testing/variations/fieldtrial_testing_config.json

@@ -2239,6 +2239,21 @@
             ]
         }
     ],
+    "EnableAutofillPasswordReauthIOS": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableAutofillPasswordReauthIOS"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableContextMenuPerformanceHints": [
         {
             "platforms": [