[signin] Enable Dice M3 in fieldtrial_testing_config

Change-Id: I4d36c589cfdbd55784d31da65083c9f2fb724bd1
Bug: 807824
Reviewed-on: https://chromium-review.googlesource.com/1071289Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569640}

testing/variations/fieldtrial_testing_config.json

 {
+    "AccountConsistencyVariations": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "DiceMigration",
+                    "params": {
+                        "method": "dice_migration"
+                    },
+                    "enable_features": [
+                        "AccountConsistency"
+                    ]
+                }
+            ]
+        }
+    ],
     "AllowClientHintsToThirdParty": [
         {
             "platforms": [