[iOS] Add fieldtrial for error page

Launch bug: crbug.com/1137329

Bug: none
Change-Id: I2c1c51a92b636c7c3685f84c1ca4e5fb4e3905dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2549706
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840570}

testing/variations/fieldtrial_testing_config.json

@@ -3752,6 +3752,21 @@
             ]
         }
     ],
+    "IOSErrorPageRefactoring": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "UseJSForErrorPage"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSFullPageScreenshot": [
         {
             "platforms": [