[signin] Enable ForceDiceMigration in fieldtrial_testing_config.json

Change-Id: Iff7379af735fbace57075d9b920a8873bb073da1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1840431Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703629}

testing/variations/fieldtrial_testing_config.json

@@ -2623,6 +2623,23 @@
             ]
         }
     ],
+    "ForceDiceMigration": [
+        {
+            "platforms": [
+                "windows",
+                "mac",
+                "linux"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ForceDiceMigration"
+                    ]
+                }
+            ]
+        }
+    ],
     "FreezeBackgroundTabOnNetworkIdle": [
         {
             "platforms": [