Reland "[iOS] Add fieldtrial config for IOSRequestDesktopByDefault"

This is a reland of ee4eb865
The original CL was reverted because some tests were broken when the
flag was enabled.

CL fixing the tests: https://crrev.com/c/2330052

Original change's description:
> [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}

Bug: None
Change-Id: I0ce43d62cf33bc52321b2584a548308fce4227db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2329325
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796393}

testing/variations/fieldtrial_testing_config.json

@@ -3578,6 +3578,24 @@
             ]
         }
     ],
+    "IOSRequestDesktopByDefault": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "DesktopEverywhere",
+                    "enable_features": [
+                        "UseDefaultUserAgentInWebClient"
+                    ],
+                    "disable_features": [
+                        "MobileGoogleSRP"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSSSLCommittedInterstitials": [
         {
             "platforms": [