Add field trial testing config for FormControlsDarkMode

See launch bug: crbug.com/1086530

Bug: 1086530
Change-Id: I059958cf508c3fc96dd3295144a55a505d837613
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358436Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Yu Han <yuzhehan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799000}

testing/variations/fieldtrial_testing_config.json

@@ -2591,6 +2591,24 @@
             ]
         }
     ],
+    "FormControlsDarkMode": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CSSColorSchemeUARendering"
+                    ]
+                }
+            ]
+        }
+    ],
     "FormControlsRefresh": [
         {
             "platforms": [