[Sync::USS] Add fieldtrial testing config for SyncUSSBookmarks

Bug: 889819
Change-Id: I102d62cf4a8e050566a17b9796abcbc56e77ea0b
Reviewed-on: https://chromium-review.googlesource.com/c/1458442
Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630558}

testing/variations/fieldtrial_testing_config.json

@@ -4612,6 +4612,26 @@
             ]
         }
     ],
+    "SyncUssBookmarks'": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SyncUSSBookmarks"
+                    ]
+                }
+            ]
+        }
+    ],
     "TabRanker": [
         {
             "platforms": [