Add fieldtrial testing config for MergeBookmarksUsingGUIDs

Change-Id: I71239f7e49bd6b2c19a6a526a2a800108154e3cc
Bug: 978430
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2077717
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747192}

testing/variations/fieldtrial_testing_config.json

@@ -5220,6 +5220,26 @@
             ]
         }
     ],
+    "SyncMergeBookmarksUsingGUIDs": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "MergeBookmarksUsingGUIDs"
+                    ]
+                }
+            ]
+        }
+    ],
     "SyncUSSNigori": [
         {
             "platforms": [