Enable Dark Mode Support by Default for Chromium Builds

BUG=927180

Change-Id: I1617453d53c9fda86db53ed5672e740ab4fb58af
Reviewed-on: https://chromium-review.googlesource.com/c/1481850
Auto-Submit: Robert Liao <robliao@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634387}

testing/variations/fieldtrial_testing_config.json

@@ -1353,6 +1353,21 @@
             ]
         }
     ],
+    "DarkModeWindows": [
+        {
+            "platforms": [
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "DarkMode",
+                    "enable_features": [
+                        "DarkMode"
+                    ]
+                }
+            ]
+        }
+    ],
     "DataCompressionProxyLoFi": [
         {
             "platforms": [