[iOS] Add fieldtrial config for IOSRequestDesktopByDefault

Bug: None
Change-Id: Iea882e82e5573edaf917d8f3d976d876e2120de9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317789
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792621}

testing/variations/fieldtrial_testing_config.json

@@ -3515,6 +3515,24 @@
             ]
         }
     ],
+    "IOSRequestDesktopByDefault": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "DesktopEverywhere",
+                    "enable_features": [
+                        "UseDefaultUserAgentInWebClient"
+                    ],
+                    "disable_features": [
+                        "MobileGoogleSRP"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSSSLCommittedInterstitials": [
         {
             "platforms": [