[iOS] Add variation for UseWKWebViewLoading

Bug: none
Change-Id: I17614a11b718a226bf00591596f378a4a3d44575
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007638
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733406}

testing/variations/fieldtrial_testing_config.json

@@ -3198,6 +3198,21 @@
             ]
         }
     ],
+    "IOSUseWKLoading": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "UseWKWebViewLoading"
+                    ]
+                }
+            ]
+        }
+    ],
     "IdentityDiscIPH": [
         {
             "platforms": [