[base] Add DisableOffSequenceTaskCancelation fieldtrial test config.

Bug: 1053628
Change-Id: I59254c7f29b05642dee3ce30c867bf190bcc18a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315032
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792690}

testing/variations/fieldtrial_testing_config.json

@@ -2285,6 +2285,25 @@
             ]
         }
     ],
+    "DisableOffSequenceTaskCancelation": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "DisableOffSequenceTaskCancelation",
+                    "disable_features": [
+                        "AllowOffSequenceTaskCancelation"
+                    ]
+                }
+            ]
+        }
+    ],
     "DnsHttpssvc": [
         {
             "platforms": [