[ios] Add Default Browser fullscreen promo config

Bug: 1107489
Change-Id: I10117e30eaf019fec202b05958af17b38133cdf8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412786
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807274}

testing/variations/fieldtrial_testing_config.json

@@ -3345,6 +3345,21 @@
             ]
         }
     ],
+    "IOSDefaultBrowserFullscreenPromo": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "DefaultBrowserFullscreenPromo"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSDiscoverFeed": [
         {
             "platforms": [