[Tab Groups] Add field trial testing config for tab groups.

Bug: 905491, 1022943
Change-Id: I2f42c75cc2948b284e09e7e8d19d3942f98a796a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991921Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729539}

testing/variations/fieldtrial_testing_config.json

@@ -2020,6 +2020,24 @@
             ]
         }
     ],
+    "DesktopTabGroups": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "TabGroups"
+                    ]
+                }
+            ]
+        }
+    ],
     "DialMediaRouteProvider": [
         {
             "platforms": [