[iOS] Add field trial testing config for Safe Browsing experiment

This adds a field trial testing config entry for the
SafeBrowsingAvailableOnIOS feature.

Bug: 1059911

Change-Id: Iaba96f738087e95d8886b54871e1177451c2e64b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137654Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757028}

testing/variations/fieldtrial_testing_config.json

@@ -4883,6 +4883,21 @@
             ]
         }
     ],
+    "SafeBrowsingAvailableOnIOS": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SafeBrowsingAvailableOnIOS"
+                    ]
+                }
+            ]
+        }
+    ],
     "SafeBrowsingCommittedInterstitials": [
         {
             "platforms": [