Add device policy to disallow shutdown - ash UI modifications

ash/system/chromeos/shutdown_policy_observer.h

+// Copyright 2014 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 ASH_SYSTEM_CHROMEOS_SHUTDOWN_POLICY_OBSERVER_H_
+#define ASH_SYSTEM_CHROMEOS_SHUTDOWN_POLICY_OBSERVER_H_
+
+namespace ash {
+
+// Used by the SystemTrayDelegate to observe the |kRebootOnShutdown| policy.

[Daniel Erat, 2015/01/07 16:18:40]

the "used by" in this comment seems a bit unclear. can you change it to
"Registered with SystemTrayDelegate to observe ..."?

[cschuet (SLOW), 2015/01/13 10:38:28]

Done.

+class ShutdownPolicyObserver {
+ public:
+  // Called when the value of the |kRebootOnShutdown| policy is changed.
+  virtual void OnShutdownPolicyChanged(bool reboot_on_shutdown) = 0;
+
+ protected:
+  virtual ~ShutdownPolicyObserver() {}
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_CHROMEOS_SHUTDOWN_POLICY_OBSERVER_H_