[ios] Add Testing Config for Fullscreen Finch Config

Adds a config for the FullscreenControllerBrowserScoped flag.

Bug: 790886
Change-Id: Ibbb765e6d7affa49bde4afc8effefdce48da94a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299236Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789252}

testing/variations/fieldtrial_testing_config.json

@@ -2956,6 +2956,21 @@
             ]
         }
     ],
+    "FullscreenBrowserScope": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "FullscreenControllerBrowserScoped"
+                    ]
+                }
+            ]
+        }
+    ],
     "FuzzyAppSearch": [
         {
             "platforms": [