Add fieldtrial testing config for SyncUSSNigori

Bug: 922900
Change-Id: I50929e2f6d467496f3a43bdebf4337a3209c8c0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871877
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713049}

testing/variations/fieldtrial_testing_config.json

@@ -6080,6 +6080,26 @@
             ]
         }
     ],
+    "SyncUssNigori": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SyncUSSNigori"
+                    ]
+                }
+            ]
+        }
+    ],
     "SyncUssPasswords'": [
         {
             "platforms": [