[signin] Revert Dice default testing config to Milestone 1

Dice Milestone 2 was enabled by default in CL:
https://chromium-review.googlesource.com/c/chromium/src/+/975561

However this is causing issues when signing into Google properties on
the web, because the new server endpoint rejects most of the client IDs.

This restores the previous behavior.

Bug: 829220
TBR: jwd
Change-Id: I3d7c22cb10f0d900c8d897369b063ac236f12146
Reviewed-on: https://chromium-review.googlesource.com/1011620
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550607}

testing/variations/fieldtrial_testing_config.json

@@ -8,9 +8,9 @@
             ],
             "experiments": [
                 {
-                    "name": "DicePrepareMigration",
+                    "name": "DiceFixAuthErrors",
                     "params": {
-                        "method": "dice_prepare_migration_new_endpoint"
+                        "method": "dice_fix_auth_errors"
                     },
                     "enable_features": [
                         "AccountConsistency"