Add testing config for iOS legacy TLS interstitial

Bug: 1100647
Change-Id: Ibd84d2d12d3b750c864311535d041064c715aa8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340642Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795743}

testing/variations/fieldtrial_testing_config.json

@@ -3533,6 +3533,21 @@
             ]
         }
     ],
+    "IOSLegacyTLSInterstitial": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "IOSLegacyTLSInterstitial"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSPageInfoRefactoring": [
         {
             "platforms": [