ozone: Allow ozone evdev/kvm features to be enabled without running on ChromeOS.

chromeos/system/devicemode.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_SYSTEM_DEVICEMODE_H_
+#define CHROMEOS_SYSTEM_DEVICEMODE_H_
+
+#include "chromeos/chromeos_export.h"
+
+namespace chromeos {
+
+// Returns true when running as system compositor. Ie. using libudev, kms and
+// evdev for input and output.
+CHROMEOS_EXPORT bool IsRunningAsSystemCompositor();
+
+}  // namespace chromeos
+
+#endif  // CHROMEOS_SYSTEM_DEVICEMODE_H_