[iOS] Adds ExtendOpenInFilesSupport finch config

This CL adds ExtendOpenInFilesSupport to the fieldtrial testing config

Bug: 1117398
Change-Id: I9d52da7c57e101b1e2607b0c9c6e700e9415468c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454075Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815048}

testing/variations/fieldtrial_testing_config.json

@@ -2755,6 +2755,21 @@
             ]
         }
     ],
+    "ExtendOpenInFilesSupport": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ExtendOpenInFilesSupport"
+                    ]
+                }
+            ]
+        }
+    ],
     "ExtensionSettingsOverriddenDialogs": [
         {
             "platforms": [