Disable guest mode by default if force sign in is enabled.

chrome/browser/profiles/guest_mode_policy_handler.h

+// Copyright 2016 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 CHROME_BROWSER_PROFILES_GUEST_MODE_POLICY_HANDLER_H_
+#define CHROME_BROWSER_PROFILES_GUEST_MODE_POLICY_HANDLER_H_
+
+#include "base/macros.h"
+#include "components/policy/core/browser/configuration_policy_handler.h"
+
+class PrefValueMap;
+
+namespace policy {
+
+class PolicyMap;
+
+class GuestModePolicyHandler : public TypeCheckingPolicyHandler {
+ public:
+  GuestModePolicyHandler();
+  ~GuestModePolicyHandler() override;
+
+  void ApplyPolicySettings(const PolicyMap& policies,
+                           PrefValueMap* prefs) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(GuestModePolicyHandler);
+};
+
+}  // namespace policy
+
+#endif  // CHROME_BROWSER_PROFILES_GUEST_MODE_POLICY_HANDLER_H_